I really have no idea which animes count as main-stream other than Naruto and Bleach. So here is a whole list of other ones I've watched that were really good. I can't really choose one either.
Code Geass
Nabari No Uo
Death Note
Elfen Lied
Gurrenn Lagann
The World God Only Knows
Sword Art Online
No. 6
Betrayal Knows My Name
Tiger and Bunny
Black Butler
Soul Eater
Angel Beats
Guilty Crown
Kids on The Slope