This is rather simple- the units that have had the best level ups in a playthrough, and how much memories you have of that particular run. Preferably your first run of something (of a game, of a difficulty, first time using that unit, etc), but if a later run had a better memory, go.
For me, I've only had the chance to play FE7 and FE8 personally.
*Florina has got to be my most blessed unit of all time, and has several miracles to her name.
*In FE8, almost obligatorily, Vanessa, and to a lesser extent Tana, have been serving me well, but by far the best unit I've had so far (the run's around the halfway point) is Ephraim. His first blow was a critical, his first level up had six stats, the rest of his level ups have had multiple well-placed points, and he's never been in a troublesome position. It says something that I gained more respect for Kyle's first level when Ephraim copied it shortly after (HP, Str, Def).