What could possible be next? I've seen all the treks. I still have Farscape, Stargate (all of them), Lost, Dr. Who... What's up the alley of a new BSG, b5, Firefly and DS9 fan?
Farscape and Lost. Probably Lost is better show overall, but Farscape is pretty unique Sci-Fi and very well done. That being said, Lost's main story may end up rivaling Bab5's amazing arc.
Stargate's are sort of mundane and nothing exceptional but they'll allow to pass the time reasonably well and may even occasionally throw in a surprise or two (though I wouldn't count on many)
Dr. Who I haven't seen.
"Heros" is pretty good.
Angel & Buffy are both excellent of course with Angel typically appealing to the Bab5/DS9 crowd more so than Buffy. Of course, if you watch Angel (and you really should), you need to watch Seasons 1-3 of Buffy first, then watch Buffy Season 4 and Angel Season 1 together, then finish one of the series straight out and then the other one.
Highlander the series is pretty good though not quite as good as your Firefly's and DS9's.
Dead Like Me is a nice short somewhat sci-fi'ish show (though those aspects aren't really the focus).