For example, in the United States, the chief executive of the federal government has, since the repeal of the failed Articles of Confederation, always been called the President, and despite changes in presidential roles from George Washington to Joe Biden, the office held by both is one in the same, with perfect continuity after more than 230 years.
However, looking at the "List of leaders of the Soviet Union" on Wikipedia, I see that the leaders' titles varied considerably until the period including and following Brezhnev. Did each leader choose his own title, or does the fact that, for example, Krushchev never had the same title as Stalin mean that they held fundamentally different positions that had shifts in relative power that became apparent only when Stalin died off and Malenko was forced to resign?
In short, why does Soviet chief executive succession seem so messy with such inconsistent offices and/or titles?
Part of the problem is that the list you are looking at on Wikipedia is obscuring the consistently similar office held by all the leaders of the country. For the vast majority of its history, the Soviet Union was lead by a person with either the title of "General Secretary" or "First Secretary" who chaired Politburo meetings and delivered the Work Reports of the Central Committee to each meeting of the full Party Congress. The only partial exception to this was Lenin from 1922-1924, and this is only because it had not yet been agreed that the General Secretary would chair the Politburo on a permanent basis. The party had no one formal leader and Lenin held leadership over the party through his personal charisma and the fact that he was consistently the highest vote-getter among Bolshevik Party Congress delegates when Central Committee elections were held. It was because of this de facto position as the party leader that Lenin assumed the position of Premier, first of Russia and then of the Soviet union when it was founded five years later, but did not signify any change within the party hierarchy as he continued to be the de facto leader of Politburo throughout this time.
To be pedantic, it is true that from 1952-1953, the position of General Secretary was technically abolished and replaced with a system in which the executive work of the party was lead by a group of supposedly co-equal secretaries, with Stalin remaining in power as "only" one of these secretaries. This was supposedly done because Stalin was in ill health and wanted to reduce his workload. Stalin was nonetheless the unquestioned leader of the party and country until his death a year later. When the position of General Secretary reestablished, it was also technically called the "first secretary" from 1953-1966, though this was only a change of nomenclature as the two holders, Khrushchev and Brezhnev, held all of the same powers of the old General Secretaryship. Brezhnev returned the title to its old form and the leader of the country was widely understood the be the General Secretary until the end of the USSR.
While it would be sufficient to say that the leader of the communist party, whether General Secretary or First Secretary, was always the leader of the country, it is also true that all of the General Secretaries also held the title of head of government (Premier) or head of state (Chairman of the Presidium) at some point during their time in power. Stalin and Khrushchev were both the most powerful person in the country for awhile before they became Premier but assumed the position later in order to consolidate power during periods of significant challenge to their leadership. After Khrushchev was removed from power, it was agreed among the Politburo that the leader of the party should not be allowed to become Premier again, as chairing both the Politburo and the Council of Ministers would give him too much power.
The precedent of not combining the positions of head of government and head of party remained until the end of the USSR, but it did not stop Brezhnev from later assuming the role of head of state or Chairman of the Presidium. Brezhnev assumed this position in 1977 after purging his longtime rival Nikolai Podgorny, who had been an opponent of many of his policies and ultimately the new 1977 Soviet constitution. Brezhnev thus took the opportunity of the new constitution's promulgation to remove Podgorny from power and take his position for himself. Brezhnev's two successors Andropov and Chernenko, followed suit in holding the positions of General Secretary and Chairman of Presidium simultaneously. The prevailing mood among the Politburo in these brief years was to avoid changing the status quo among the remaining elder leaders by splitting the positions.
When Gorbachev succeed Chernenko, the Politburo agreed to break precedent and split the positions, potentially because of Gorbachev's relative youth and the fact that his assumption of the General Secretaryship was far from unanimous. Instead the Chairmanship of the Presidium was given to the veteran diplomat Andrei Gromyko. Gromyko's retirement in 1988 nonetheless paved the way for Gorbachev to succeed to the position anyway. The Chairman of the Presidium was renamed the "President of the Soviet Union" in the USSR's final year as part of Gorbachev's failed effort to create a unitary executive branch. It was this position that Gorbachev held alone after the Communist Party, and the General Secretaryship with it, was abolished, leaving Gorbachev to end the USSR with his resignation as President.
It isn't a messy as it looks at first glance - there are really only two titles there: premier/prime minister, and leader of the (communist) party. A third title should be there in 1990-1991: president.
The premier/prime minister was the official head of the Soviet government. However, since the Communist Party was the ruling party, the party leader could be a candidate for supreme power. There is also the position of head of state, but this was never a real leadership position until the very end of the Soviet Union.
The most straightforward is the position of premier or prime minister. The Soviet cabinet (i.e., the ministers) was originally called the Council of People's Commissars (Soviet narodnykh kommissarov, abbreviated to Sovnarkom) and later the Council of Ministers of the Soviet Union, and finally in 1991, the Cabinet of Ministers. The chairman was effectively the prime minister or premier (and is/was often called by one of those titles in the West). The history of the chairmanship of these bodies is:
Council of People's Commissars of the Russian Soviet Federative Socialist Republic (before the Soviet Union formed)
Council of People's Commissars of the Soviet Union
Vladimir Lenin 1923-1924
Alexei Rykov 1924-1930
Vyacheslav Molotov 1930-1941
Joseph Stalin 1941-1946
Council of Ministers of the Soviet Union
Joseph Stalin 1946-1953
Georgy Malenkov 1953-1955
Nikolai Bulganin 1955-1958
Nikita Khrushchev 1958-1964
Alexei Kosygin 1964-1980
Nikolai Tikhonov 1980-1985
Nikolai Ryzhkov 1985-1991
Valentin Pavlov 1991
Cabinet of Ministers of the Soviet Union
Valentin Pavlov 1991
Ivan Silayev 1991
The party leadership was more opaque. The official body of the party leadership was the Central Committee of the Communist Party of the Soviet Union. The other party body making political decisions was the Political Bureau of the Central Committee of the Communist Party of the Soviet Union (Politburo), temporarily remade into the Presidium in 1952-1966. Under Stalin, the Politburo replaced the Central Committee as the main decision-making body of the party (this eclipse of the Central Committee by the very pro-Stalin Politburo was one of the mechanisms of Stalin's takeover of power). The other two important party bodies were the Secretariat of the Communist Party of the Soviet Union (responsible for the day-to-day running of the party) and the Organisational Bureau of the Central Committee of the Communist Party of the Soviet Union (Orgburo), responsible for organisation and personnel (which meant that it was a way to place personal supporters in key posts). The Orgburo was absorbed by the Secretariat in 1952. The General Secretary tied these organisations together. Officially the General Secretary of the Central Committee, and originally elected by the Central Committee (and later the Politburo), the General Secretary began as an administrative role (under Lenin), and became the de facto party leader (and de facto head of government, with the premier as the de jure head of government) under Stalin. The Stalin-and-later General Secretary chaired the Central Committee, and was the head of the Secretariat and Orgburo. The General Secretary usually chaired the Politburo, by custom rather than rule (the only exception that comes to mind is Malenkov continuing to chair the Politburo into 1954, after Khrushchev had become First Secretary (as the General Secretary had been renamed, until 1966; this was a straightforward de-Stalinisation change, and logical since the next most senior position in the Secretariat was the Second Secretary)).
After Stalin, the pathway to leadership was through the Secretariat and Politburo. All of the post-Stalin leaders of the Soviet Union were members of the Politburo and senior members of the Secretariat, and almost all of the post-Stalin leaders are listed on that Wikipedia list as General/First Secretaries. The exception is Malenkov, temporary leader in the post-Stalin interregnum. He was briefly General/First Secretary, at least in an acting capacity if not officially, since he was Second Secretary when Stalin died. However, in a reaction to the long concentration of power in one man's hands under Stalin, he was forced to resign from the Secretariat, reducing his official role as leader to Premier (thus, his title in the Wikipedia list).
Finally, the official head of state: The highest legislative body was the All-Russian Congress of Soviets (1917-1922), the Congress of Soviets of the Soviet Union (1922-1938), the Supreme Soviet of the Soviet Union (1938-1989), and the Congress of People's Deputies of the Soviet Union (1989-1991). This last body grew from the 1,500 of the Supreme Soviet to 2,250, and was accompanied by a new smaller Supreme Soviet of 542 deputies. The chairman of the Central Executive Committee of the congresses was the official head of state. The Supreme Soviet elected the Presidium of the Supreme Soviet (consisting of a chairman, vice-chairman, secretary, 15 deputies (one per republic) and 20 members), which was a collective head of state, but the chairman functioned as an individual head of state. The chairman of the final incarnation of the Supreme Soviet was the head of state. Finally, the position of President of the Soviet Union as a head of state was created in 1990. The one and only president was elected by the Congress of People's Deputies, but future presidents would have been elected by popular vote if the Soviet Union had remained in existence. There had been a suggestion, rejected by Stalin, to have the chair of the Presidium elected by direct popular vote. The heads of state were:
Chairman of the All-Russian Congress of Soviets:
Lev Kamenev 1917-1917
Yakov Sverdlov 1917-1919
Mikhail Vladimirsky 1919-1919
Mikhail Kalinin 1919-1922
Chairman of the Congress of Soviets of the Soviet Union
Chairman of the Presidium of the Supreme Soviet
Mikhail Kalinin 1922-1938
Nikolay Shvernik 1946-1953
Kliment Voroshilov 1953-1960
Leonid Brezhnev 1960-1964
Anastas Mikoyan 1964-1965
Nikolai Podgorny 1965-1977
Leonid Brezhnev 1977-1982
Vasili Kuznetsov 1982-1983
Yuri Andropov 1983-1984
Vasili Kuznetsov 1984-1984
Konstantin Chernenko 1984-1985
Vasili Kuznetsov 1985-1985
Andrei Gromyko 1985-1988
Mikhail Gorbachev 1988-1989
Chairman of the Supreme Soviet
President of the Soviet Union
It was only at the very end of this role of head of state, under Gorbachev's presidency, that it became a genuine leadership role. In 1990, the Communist Party lost its official role as "leading and guiding force of the Soviet society", and Gorbachev was leader of the Soviet Union through being president rather than General Secretary (and resigned this last role in 1991, briefly replaced by Vladimir Ivashko, the only General Secretary from Stalin onwards to not be leader of the Soviet Union).