List of really good ones:
-op for cowboy bebop
-op & ed for elfen lied (the ending is kinda gay, but it really connects with the story IMO. And the song isn't necessarily bad.. just not something to have on my Ipod)
-op & ed for evangelion
-op for higurashi no naku koro ni, and higurashi no naku koro ni kai
-ed for higurashi no naku koro ni
-op for digimon 1 and 3 in spanish or japanes (The songs are bad ass so stfu)*
-op for pokemon (you know it)
-op for Lucky Star, and don't you know it (if you think it's gay, it means you are dumb enough not to get the point)
-op for Clannad After Story
-ed for wolf's rain
Song's that get stuck in my head even though the songs are not my style at all. I like the songs, but only when I watch the anime... not in my ipod.
-op for toradora!
-God knows from The Melancholy of Haruhi Suzumiya (the one she sings in the concert. The song is actually pretty cool. The guitar riff is simple but really badass sounding)
-op and ed of TMoHS
-ed of Myself, Yourself
-op for Kanon
-op for clannad (the ending song gets stuck in my head :S)
-The new ed song for the rebuild of evangelion
Naruto and Ranma 1/2 have really cool or catchy op (not all of them) but I don't really want to list them.