What is the weakest Pokémon (stat wise) (not level wise) you've taken on a Champion with?

My Sunbo the Sunflora in my SoulSilver version. It held it's own quiet well surprisingly.

I've taken a level 100 Magikarp through the Elite Four, with the aide of only one other Pokemon. Granted, that Pokemon was a hacked Spiritomb with Wonder Guard that I got over the GTS, but I felt I had to at least use them a bit before I release them both (and now they're in the Hall of Fame forever!).

Other than that, I have used Butterfree as my main Pokemon numerous times against the Elite Four, which stat wise, is worse than a lot of middle evolutions of Pokemon let alone final ones.

I haven't really thought about it, but that spot would probably go to Mightyena who I've used in almost all of my Hoenn playthroughs just because I love it.

I just finished the storyline of my Black 2 on my 3DS, my Lopunny wrecked Iris' team(with the help of some potions lolololol)

:t189: Jumpluff. Everyone forgets these things exist but I love them!

Well, when I did the Christian Challenge, I had a team of mainly NFE Pokemon, so that would include Torchic, Clamperl, Jigglypuff, and ummm I know I'm forgetting someone. Oh well. I've also used Petilil and Delibird in Black and Eevee in FireRed.

Haunter, on a solo challenge I did, on Gen I.
It's such a shame I couldn't evolve it :(

mmm... used alot of weak pokemons over the year(I'm just choosing pokemons based on look not power).
I guess it would be Masquerain on Ruby, their might be even weaker but for now I don't recall any.

Shuckle, only thing you can do is bide. If you stat swap for amazing damage, you're bound to get 1 hit as he has such low hp. No matter how you look at it, Shuckle can only be used effectively defensively.

Definitely my Roselia back in my Ruby version, she was one heck of a Pokemon that definitely held her own in battle so yeah she really surprised me at how well she did, but then again she wasn't part of an evolution line back in those days.