At one point in time I might have said Jirachi, which is definitely up there, or maybe Swampert, but considering that my GBA SP had (or, well, has) a puff-paint Latias on it, and I have a Latias figure sitting on my shelf just a few feet away, and I have a Latias plushie on my nightstand, and when I was a kid I went out of my way to get one of my dad's Pokémon friends to get me the Eon ticket so I could catch Latias in my copy of Ruby (even though I later also got a copy of Sapphire and could absolutely have beaten the game, caught it, and traded it over), and I watched Pokémon Heroes a lot as a kid (oh hey I just realized that Bianca from that movie is gender, and guess which Pokémon disguises itself as her on the regular), and also its head kinda reminds me of Zero's helmet...
I think I'm gonna say Latias 
(In fairness I do also have a Jirachi plush and figure in those same locations, and that lil' fella carried me through basically all of Ruby because I got it through my brother's Colosseum bonus disc, but Latias has a bit more going for it that pushes me in that direction)
... As far as non-Pokémon creachers go, I will never not find it hilarious that Zinogre's moveset is based on Terry Bogard, and also it's just a cool monster and a fun fight.
Oh, and also I once devised a playable D&D species based on Moosh from Zelda (I still need to get around to playing the Oracles games, oop) called arcturians, which remains one of my proudest creations from when I was still making 5e homebrew