My first one was technically my Lv. 100 Blaziken in Ruby, but that's because I was an idiot and rare candied the crap out of it. So I suppose it's not "legitimate" in the sense of "I spent hours grinding to get it max level", but still.
After that... I honestly don't even remember. x.x It's been such a long time since I've had any Lv. 100 Pokemon at all, not including events.
Following the common trend, it was a starter and sticking to the 90s kid stereotype it was a Charizard. Except mine was in X rather than Gen I. Until recently it was very hard for me to get any Pokémon to level 100 and even now it’s a pretty rare occurrence.
Technically, my first level 100 Pokemon was my beloved Espeon in Pokemon Silver. But I did use the cloning glitch to duplicate Rare Candies to get it to that level.
So, my very first Pokemon to reach level 100 without glitches was my Sceptile in Pokemon Ruby. She was the star of my game back in the day and I'm very sad that I lost her along with the cartridge so many years ago.
Reshiram and Zekrom, it's because I wanted their level 100 signature moves and to match with the rest of the trio, I also leveled up Kyurem to 100. I'm never giving up those three no matter what and I will use them in any battle I want.