Inserts are sometimes better than the OP and ED themes. You watch a sequence and whoa, you hear a really good song. So far, some of my favourites are:
Akatsuki no Kuruma from GS
Perfect World from Escaflowne
Hatenaki Monologue from Soukyu no Fafner: Right or Left
Winner from FY
Namida no Riyuu from School Days
Your Gravitation from Seto no Hanayome
Lunarian from Seto no Hanayome
Happily Ever After from Tengen Toppa Gurren Lagann
What's yours? Thanks!