Pokémon Yellow for me. Or, well, it was my cousin's game, but we always played it together. We got an Alakazam up to lvl. 100 after many weeks of grinding (or so it felt like). It rocked the Elite Four with Psychic, Psybeam, Recover and Mega Kick. What a lovely time that was.
Later, on his Silver game, we had this awesome Heracross with Earthquake, Megahorn, Reversal and some other move I can't remember. It absolutely wrecked everything in the game, even lolstomping Red's team.
The first game I owned myself was Crystal. Can't remember which Pokémon I got to lvl. 100 first, but I guess it was a Dragonite. Being traded from my cousin's game, it grew faster than the others, and it wasn't as painfully slow to train as the rest of the 'mons in Crystal. After all, grinding in GSC was really tedious because of the abysmal levels of all in-game opponents apart from Red (seriously, Lance had ONE Pokémon in lvl. 50).
Since then, it's been a few, across virtually all games released since RSE. None particularly notable, they've mostly been Pokémon I've EV'd, trained and then powerlevelled until lvl. 100. I've never filled a full box of them in any one game, but altogether, I might have roughly 50 of them by now.