Mine would be Flygon, easily.
In every game, even going back to when I was a kid playing Gen 2 and 3, I always avoided using overpowered Pokémon. Don't know why really, but I just always preferred to use the more underrated ones. And early on, this meant I never used dragons (Dragonite, Salamance, even Charizard). BUT then there is Flygon, who is the first dragon who didn't feel like it was meant to be OP, with an incredible design and color scheme. And to this day, In whatever game, if Flygon is available, it's on my team :)
I never nickname my Pokémon unless im doing a challenge, so probably no nickname.