Oh man, I used to be an OP/END theme JUNKY. Seriously. I would make mix tapes of nothing but op/end themes to listen to on my walkman whenever I walked anywhere. (I still have lots of them on my ipod now, who am I kidding?! orz)
I love the ones already listed...here's some more faves:
"Kimi Iro Omoi" from Akazukin Chacha:
http://www.youtube.com/watch?v=KHzYAPvQPXs My first exposure to SMAP! I still adore this show, don't let the fact that it's technically a magical girl show fool you, it is HYSTERICAL.
"Chacha ni omakase!" from Akazukin Chacha:
http://www.youtube.com/watch?v=N4ImD4RG6SU Probably my favorite ending from the series, it's just so catchy and fun!
"Koi wo Suru tabi ni Kizu Tsuki Yasuku" from "Nurse Angel Ririka SOS":
http://www.youtube.com/watch?v=ds3xZ5R30H8 I always loved this one because it had such an 80s dance sound to it, y'know?
"What's up guys?" from Sorcerer Hunters:
http://www.youtube.com/watch?v=95MS5NJs5w0
"Mask" from Sorcerer Hunters:
http://www.youtube.com/watch?v=Mb0rF9ktX90
I love all the music from Sorcerer Hunters, it really did have an awesome soundtrack.
"Rusian Roulette" from Dirty Pair:
http://www.youtube.com/watch?v=JNd6SpmeBqY oh man I still listen to this frequently, I just love it. XD (I need to get those new DP box sets eventually.)
As much as I LOVE "Lum no Love Song", I think some of Urusei Yatsura's other OPs get overlooked a lot, such as:
"Dancing Star"
http://www.youtube.com/watch?v=ZZ5gi9PH7j0
"Chance on Love"
http://www.youtube.com/watch?v=1B5AFdnhJVM
and "Tonogata gomen asobase!"
http://www.youtube.com/watch?v=Sc92Qq7Ruog
And I pretty much love every incarnation of "Honey Flash!" to ever exist, but the "New Cutey Honey" version's one of my favorites:
http://www.youtube.com/watch?v=_iKpEYyTZlo
Ok, I could probably do this all day so I'll stop here. XD Maybe I'll post more later~