I can't narrow it down very well, so instead I'm gonna do my best to narrow it down per gen.
1st: Either Bulbasaur or Eevee. Eevee is cause my best team ever was a team of all Eeveelutions I hand raised from lvl 1(which took out my friend's hacked legendary team with ease

) and Bulbasaur is cause my first playthrough of Blue I beat the entire game with just my Bulbasaur, and he was lvl 100 before I fought the Elite 4. It was magical
2nd: Typhlosion easy. There is no such thing as a better starter than Cyndaquil, and Typhlosion will destroy whatever gets in his way. He's a boss, plain and simple.
3rd: Milotic. Best water type period, so worth it to get one, and it just looks so purty. Seriously, give it Surf, Ice Beam, and whatever other special attacks you wish and watch it dominate.
4th: Glaceon. I really enjoy using Ice types and/or moves, and Glaceon delivers those with astounding power. Tie with Espeon for highest Sp. Atk of the Eeveelutions while also being able to take more than one hit? Yes please!
5th: Hydreigon without a doubt. It's a beast in battle, looks awesome, and is a DARK DRAGON. Seriously, what more do I need to say?