Well I'll just go down the list:
Avatar would be fun. Probably wouldn't get to be an Airbender(my new favorite) but it would be fun to bend fire too. Or I could be an awesome nonbender like Ty Lee or Jet. Fun exciting place.
Bleach would be fine enough since I could probably end up in the Shinigami corp. Fighting for life would be fun and I'd make my Zanpakuto work. If Ikkaku can figure it out, can't be that hard.
Campcom is broad but if I can stay in the Battle Network world, it would be okay. Fighting Netbattles for life=fun.
Cartoon Network would be really fun but really for nostalgia. Short of getting some of Dexter's tech or KND stuff, I really wouldn't be able to have as exciting adventures. Though I could come up with an awesome imaginary friend.
Dunno Dark Horse but DC would leave me forever in the shadow of Supes/Batman. That and I'd suffer a bad case of can't catch up.
Death Note only if I can get a Death Note. Otherwise why bother?
Digimon would be cool. I know the Digimon I'd have/transform into(season 4).
Disney is a wonderful world of magic so I'd be able to live with it.
Dunno the next two but DBZ would pretty much leave me nigh irrelevant. I guess I could chill around Capsule Corp and wait until they summon a Dragon but eh.
Final Fantasy depends on the game but overall I could live with it. 7 in particular since Materia has a lot of potential.
Dunno Firefly but Gundam always depressed me as a child. Pass.
Harry Potter would be fun, cept 1/4th of the school is apparently racist. And if I have to put up with "likeable good guys" like Sirius Black, then I might just pass.
Dunno Heroes, or Mario really. Marvel is the same as DC, just in some other guys shadow.
Naruto would be fun. I'd engineer a way to be Edo Tensei'd back to life, break free, and then spend the rest of my revived life learning every jutsu ever. And boning Hinata/Sakura(or trying to. I'm pretty fail and stuff).
Nickelodeon solely if I can retain my knowledge from the shows. I'd capture Cosmo/Wanda in a heart beat and eventually conquer the world with their power.
One Piece would also be fun. Learn to Haki and get Vegapunk's book on DFs. Then go and find the best one I can and use it. No great ambitions but I would attempt to take control of Alabasta(by just being a nice guy to Vivi and stuff).
Pokemon would be cool but would require a lot of work. I would take it though.
Power Rangers would be fun. Dunno if I qualify as a teenager with attitude though.
Dunno about the next three at all but Star Wars depends on how many awesome blood cells I have...so pass.
Narnia I could honestly live in. Nice place, nice magic, and yeah, definite afterlife for those content to be a nice person. Might get boring though since Narnian's aren't exactly the most entertaining fellas.
Dunno Stargate but TMNT would be fun, particularly the 2003 version. Learning awesome ninja techs would be fun. I could deal with it.
Warner Bros....that's vague. I'll pass. Yugioh on the other hand might be nice, so long as I steer clear of the guys with millennium items.
And dunno the next four. I could go by the one Transformers cartoon I've seen but my memory is hazy of it. Though I did enjoy watching it.