Unless I'm mistaken, most Roman emperors would have only been "Imperator" to the soldiers. And while we still do use "Prince" and "Lord (cognate to Dominus)", they represent something far less prestigious than "Emperor". We even apply the term to post-Roman rulers who are not marital in character. Why has the title of Imperator retained such unique prestige above all others?
I think it's prudent to note that it depends on language. For example "Kaiser" and "Tsar" from Germany and Russia respectively are their words for "Emperor", both of which stemming from the Latin emperor title "Caesar". This naming convention is especially prominent in Central, Eastern, and Nothern European countries such as ones in Scandinavia, the Balkans, and Poland, Hungary etc.
Titles influenced by the term "Imperator" are mostly in Western Europe and Italy. I'm not quite sure why this is. But I will say that, after the transition from Republic to Empire, Imperator became a title exclusive only to the Emperor (likely due to the prestige and might the title conveyed) and a soldier calling one's general an 'Imperator' it was seen as treasonous. So Imperator basically became synonymous with "Augustus" and "Caesar".