Moved to the Brawl forum... lol
I'm not very knowledgeable 'bout Red and his Pokes, TBH. Normally I'd have somethin' to say 'bout Brawl's metagame, but... Nothing. ;_; All my knowledge of the three is extremely outdated.
Though, if I had to guess, I'd say Charizard. All of their recoveries are quite gimpable, but at least Charizard's got multiple jumps, a glide, and Fly takes 'em further than Waterfall and... uh, Vine Whip, is it? Ivysaur and Squirtle both have trouble finishing their food when Charizard has very suitable KO power. The only exception is if the blue turtle does some WoP gimping, and Charizard has a meteor (D-air) for gimps regardless. I recall his F-air being pretty nasty for some off-stage fighting with it's horizontal knockback. Without proper DI, it is horizontal/slightly diagonal downwards, right?
Ivysaur might get my vote if he didn't have a bloody tether, and more reliable KO power. I know from experience that his approach game and B-air WoPing is damn nasty to deal with. Squirtle's lack of KO power is what puts me off. If it ain't fresh and hitting a foe at a ridiculously high %, it ain't a KO.
Then again, I like characters that aren't terribly safe for approaching like Charizard, so... bias, perhaps. :P