Stalin's first meeting with Lenin is noteworthy:

The two met in December 1905 at the Third Congress of the Russian Social Democratic Workers’ Party in Tammerfors, in Russian-ruled Finland, where Jughashvili [Stalin] was one of the three delegates of the Bolshevik faction of the Caucasus. Lenin had returned from Swiss exile to Russia only in November 1905, having chosen to sit out most of the revolutionary events of that year. Just shy of thirty-six, he was nearly a decade older than Jughashvili. (The “patriarch” of all delegates, Mikho Tskhakaya from the Caucasus, was thirty-nine.) But Jughashvili observed at the Party Congress how provincial delegates, himself among them, attacked the elder Lenin’s policy proposals and how the Bolshevik leader backed down, rationalizing that he was an emigre out of touch. “I expected to see the mountain eagle of our party, a great man, not only politically but physically, for I had formed for myself a picture of Lenin as a giant, as a stately, representative figure of a man,” Stalin would recall. “What was my disappointment when I saw the most ordinary individual, below average height, distinguished from ordinary mortals by, literally, nothing.” (Stalin’s writings between 1906 and 1913 would contain a mere two citations of Lenin.) Eventually, of course, Lenin would become Stalin’s indispensable mentor, but it would take time for the Georgian— and most everyone else on the left— to appreciate Lenin’s history-bending force of will.

For his part, Stalin did not make an enormous initial impression on Lenin, either. Years after their first meeting, Lenin wrote to another colleague asking if he remembered the real name of their old friend "Koba" (one of Stalin's earliest revolutionary pseudonyms).

Source: Stalin: Volume I: Paradoxes of Power, 1878-1928 by Stephen Kotkin