Squirtle by far is the most annoying character to me. And I'm a Ivysaur main. I've played vs Sonic users, Olimar users, Game and Watch, Zelda and Links. While all of them are possibly incredibly annoying, none of them have annoyed me to the point that squirtle does.
Side-B, if the person isn't a noob with it, it's fairly hard to react to (on wifi). There's a slim chance to grab them out of it, in order to punish them hard. However your most basic approaches are quite literally stumped by his Neutral B and side b. Any of his smash or regular attacks that use his shell has super armor. Seriously? A dude moving that fast and agile with super armor? There's a reason that most of the super hitters have super armor. But squirtle?
His bubble instantly makes you trip. Nuff said.
His Up-B is annoying as hell to try and stop. If you try to stuff it with a aerial Up-A for the knock down, he has a shield of water around him that protects his hurtbox from your hitbox. If you try to punish it, you get hit by the water explosion.
Random Super Armors, check. A multi-hitting neutral air, check. A move that makes you trip, check. Spaztastic moves, jab , down-tilt, forward tilt, check. A throw that does a ridiculous damage, check.
Unbeatable? Not at all. But Squirtle just seems to be one of the few characters that just drains the fun out of the match for me. You can argue that Sonic is just as annoying, but Sonic doesn't have a super armor. Just imagine if Sonic's spindashes gave him a super armor. You'll pretty much have squirtle.