If I ended up in Haruhi, I'd try to join the Sos brigade, but first I'd have to be interesting. Maybe coming from another world would pass, or I'd have to be a superhero or something like that. Then I'd compete with Kyon (he's pretty much with Haruhi and very protective of Mikuru, so that leaves me with Yuki. Not that that's a bad thing, mind you).
Or in Nanoha, I would either be completely in the background or somehow develop magic powers. I'll just hope it's at least later in the first season, the show really didn't start out too well. And I'd hit on Signum since Fate is taken (not explicitly, but come on, it's obvious), or (please don't kill me) Vita (she's more than a century old, so it's ok).
As much as I insist Half Life 2 is the best FPS ever, Halo seems like the fun one to be in, thanks to several experiences playing co-op with a friend. Both would be awesome, of course.
And I'd like to be in Evangelion as the sane guy who helps and cheers up everyone. After making most episodes end with learning some aesop rather that just spiraling into further depression and also having fun with the cast, maybe Shinji would get in the eva and kick ass and chew bubblegum instead of being finally broken by the universe around him. I'd play epic music in the process and shoot some dudes, of course, and maybe start something with Asuka (I'm 15, it's ok). She would end up with newly-badass Shinji in the end, however (who else would he end up with, a clone of his mother and the angel mother of humanity?), as a fitting conclusion to what is now a series with a happy, comprehensible ending.
Sorry for the long post, but the OP just had so much potential. And the eva thing at the end is a running fantasy of mine.