Taking time to view each and every opening for each and every anime airing in Fall 2008, this list is not simply taken from the shows I'm currently watching. The only cutoff is that the OP has to have just begun this season.
There are a lot of good OPs this season, with many vying for the top spots. The ordering could easily change on a whim, but not ranking them would be the sissy's way out!
Update (12/07/2008): And speaking of that whim, I'm now flipping the order around, adding two more to the list (now 12 OPs).
12) Rosario to Vampire Capu2
"DISCOTHEQUE" by Nana Mizuki
11) Toradora
"Pre-Parade" by Rie Kugimiya, Eri Kitamura, and Yui Horie
10) Kannagi
"motto ~ Hade ni ne!" by Haruka Tomatsu
9) ef - a tale of memories
"ebullient future" by TENMON featuring ELISA
8) Michiko and Hatchin
"Paraiso" by SOIL & 'PIMP' SESSIONS
7) Linebarrels of Iron
"Kitei no Tsurugi" by Ali Project
6) Gundam 00: Second Season
"Hakanaku mo Towa no Kanashi" by UVERworld
5) Bleach
"Velonica" by Aqua Timez
4) Soul Eater
"Papermoon" by Tommy heavenly6
3) A Certain Magical Index
"PSI-missing" by Mami Kawada
2) Shikabane Hime: Aka
"Beautiful Fighter" by angela
1) Clannad ~After Story
"Toki wo Kizamu Uta" by Lia