Pikachu is freaking awesome, he/she/it/potato doesn't get nearly enough respect. I've heard varying things on sonic, mostly his crazy speed and lack of priority. PT is naturally a mixed bag.
fix'd
I think the reason pika isn't widely recognized is that none of the original melee pros who got brawl early ever really took on pika. I mean, we heard good things about it, of course, mainly the thunder and dsmash buffs, seeing as they obviously tried all the chars, but it took until after the USA release for the QAC to be brought to the attention of the public, which is really a defining feature of pika's game, imo. Pika has arguably one of the best approaches in the game because of it, and it's also huge for comboing and getting off quick thunders before they fall too low to be killed by it, for when they DI a good distance away from you after an up or down smash. And then with stuff like the reverse thunder being discovered for edgeguarding, pika is certainly evolving into quite the beast. Also, pika's thundershock was never really recognized for being as spammable as it is - imo it's better than lucas's pk fire, which is one of the main things people talk about when they mention lucas being so good, simply due to how much DI you can get in the air when you full jump or short hop it [the thundershock], making it great both for an approach and a retreat (it also gimps a bunch of characters' recoveries). And then the fair is also a very nice fast high priority combo/approach move, as well. Pika has some pretty mean combos, chiefly thanks to the incredibleness of the dsmash and thunder, and I've already pulled a couple 0-deaths with him. Pika's short hop is actually pretty short, more like melee short hops as opposed to brawl short hops, and he has good running speed. So yeah, pretty much what makes pika good right there. I don't know how well he'll rank compared to beasts like TL, olimar, or pit, but he's at worst high tier material.
As for the others... PT is actually pretty good. Squirtle is amazing, with great combo ability, speed, and kill potential. Charizard is surprisingly fast, has great power, and is in general a tank with good combos. Ivy is probably the worst of the three, but killing with usmash at 60% is sweet. Overall, PT would make a solid choice for a main, but it would require you to learn 2-3 characters at once, which you may or may not want to do.
Sonic... depends. He's got insane speed, which you'll absolutely need, due to his poor priority. He can combo, and he does have ways to kill. But whether he'll be the new falcon or a total wash... we'll have to wait and see.
Sorry the others aren't nearly as indepth as what I said about pika, but I main pika. I don't even play either of the others (although I'll probably pick up PT later down the line).
@cooper: Projectiles. Projectiles. Projectiles. And TL's fair is a nice kill move, and it ends in the air if you do it right. Fsmash, too. Not sure how much start up the fsmash has, but you should be able to get it off after a laggy attack by ike regardless. Also, dash canceled dsmashes are nice (run toward your foe, let go of forward and tap shield, and you should continue your forward momentum, but you'll be able to do anything out of it).