I'm actually obsessed with the B5 music. Chris Franke's work here is marvelous.
I recently found a site containing downloads with a lot of music playing in the series, plus intros and endings. I'm not sure if it's legal and so, actually o.o I mean, since there's a CD with (at least some of) the tracks on it? But it was a nice finding. I can't post the link yet, as the site kindly reminded me when I tried to post this post xD
My favorite intro theme is absolutely the 1st season's. It gives just the right feeling for the show. Then I've seen that the second is a little more dramatic, the third almost ridiculously dramatic and the fourth... well. Imagine. Or maybe you even remember it? It's like a mix of the 3rd and 2nd themes. Where did the beautiful 1st theme go...
The 5th theme is nice to, imo. Though it's totally different. The season is different too though, so maybe that's just ok. If you should only remember ONE of the intro themes though, it should definitely be the 1st, it's the original, in a way. I've seen many ppl say they like the 3rd the best though.