Sorry guys, I'm REALLY not trying to come down on Charizard and Ivysaur, I'm really not. They're awesome, and I love them, and frankly Charizard is my favorite of the three to use, the one I'm most comfortable with. I've said it before, and I'll say it again...they're extremely important to the TEAM, and yes, they do have some very useful match ups (and I'm TOTALLY with you on the Marth/G&W ones, from experience, especially Marth).
You have to be careful about calling them complex though. Charizard does have the most to offer in versatility (thus my "aggressive/defensive combo-style" remark), but he still has his limits, even outside of size. He's one of the few characters in the game to have such a non-defensive neutral-air (that's only situationally useful on offense), his only real sex-kick is his up-air (???), his spike requires at LEAST a full-hop to auto-cancel, and his down-smash's exceptional range is countered by being one of the laggier ones on both ends (not to mention completely incapable of hitting even briefly aerial opponents). You can pseudo-camp, you can approach, you can retreat...you've got options, but there are limitations.
Ivysaur is surprisingly beastly too, but the surprise will only last so long against people, and Ivy's limitations are WAY more noteworthy. We don't have to talk about the tether, we're all aware of what can and can't be done about it. What we can talk about it Ivy's killing. Until uber-high percents (130%), Ivy's easiest landed kill is the forward smash, which people will anticipate, followed by the up-air, which people will also anticipate, and then punish on a whiff because of the nasty end lag. Ivy's only edge-gimping consists of following a failed horizontal kill to the edge, and then harrassing with b-airs, which only really works at high-percentages. Vine Whip definitely works, but even pros don't seem to expect it, and that's the thing...they'll notice the start-up lag eventually, and adjust to the point where only a dangerously close travelling-tip is going to hit. Thank God for Bullet Seed then, but as you said in the Bullet Seed topic, people are even DIing the launch hit now. How long before it becomes a rarity that you'll actually trap a skilled opponent in the barrage for more than 15%? As far as the up-smash is concerned, that (almost) reaches Falcon Punch levels of awesome on successfully landing it, since it requires some ****ed good trickery to land. It's simply unreliable.
The thing about Squirtle though is that he has more POTENTIAL. The others simply cannot fake out their opponents the same way Squirtle can. A Squirtle with ninja mindgames can overcome his weaknesses, at least on the ground, whereas Ivysaur can be outcamped and is always at risk off-stage, while Charizard can't ever stop being big as hell (we don't want him to, though). He truly has the least insurmountable limitations.
The problem for most people seems to be frustration with Squirtle's range deficit, or predictable aerial spam, and getting hit/shield-grabbed as a result. Approaches are difficult when you're predictable, because you simply don't have the range to succeed with even mid/close-range fighting...you have to be in the thick of it, hitting with aerials and tilts and jabs and grabbing every chance you get so you can do more of that stuff. You can't smash reliably, but so what? That's why shellshifting is in the game, and as long as you keep your hydroplaning to yourself until necessary, you've got one ridiculously strong, fast, ranged KO option that can come out of nowhere. Shellshifted d-smashes have really deceptive range, too, especially since they get the slight movement boost on a shift. It's all about being fast, having excellent control, and mixing it up. You've generally got the priority, and almost always the speed, and you're small. You shouldn't be getting grabbed because of that (even in the face of irritatingly long and quick dino-dragon tongues).
I'm telling you guys...you're having an easier time with the other two because Squirtle is tougher to master, but he's really rewarding. This isn't a case of "Oh, Brawl Falcon is just as good as Melee Falcon, you're just not using him right," it's a case of "Squirtle's so good that you're having a tough time using him right." I'm not good with him, but I understand how to be, and I'm learning. Avoid predictable aerial spam, and predictability in general. I'm not totally surprised I have to reiterate and further flesh out my point...poor Squirtle is the least loved on this board. I use (and appreciate) all my Pokés equally, I just know that Squirtle has insane abilities, that NOONE ELSE IN THE GAME HAS, worth mastering.
And seriously...Pichu? Are we serious? o_o Even with Melee speed, wavedashing, and l-cancelling, Pichu ain't got **** on Squirtle. 'Sides, I don't remember the last time I saw Squirtle TAKE DAMAGE when using d-smash, up-smash, up+B, or B.