I don't have a favorite.. Just alot of good ones..
Here is some of them:
Outlaw Star:
Through The Night (OP)
Hiru no Tsuki (ED1)
Tsuki No Ie (ED2)
Rurouni Kenshin:
Sobakasu (OP1)
Tactics (ED1)
1 Third/No Junjou Na Kanjou (Not sure but I think it was the 4th ED)
Read Or Die TV:
Moments in The Sun (ED)
Fullmetal Alchemist:
Melissa (OP)
That was just a few of the songs i like..
There is soooo many good.. I'd never be able to pick one that is better than the others.