Danoz, you're watching everything that I've been watching the past few years! Here's some series that I love, I'm not sure whether you feel like just sticking with sci-fi or whether to branch out eventually, which is what I did after total sci-fi overdose (was watching far too much Heroes, BSG, ENT, VOY, DS9 etc...)
Buffy/Angel: If you're enjoying Firefly then these two shows are also great, written with similar sassy dialogue and full of character/plot arcs. Angel is technically the better show but watch Buffy too to experience total joy.
Futurama: Not a drama but excellent quality, sort of picked up where The Simpsons left off when it's own quality dropped somewhat. Really funny and more internally consistant than The Simpsons. It's also in the middle of a direct-to-DVD revival which is good for a little more longevity.
Red Dwarf: Another comedy, British this time. Really takes the piss and doesn't take things seriously - another solid show. Full of British humour (or since I'm from England, normal humour.
Heroes: I'm watching season 3 now which seems to be making up for season 2's failings somewhat, but season 1 was a brilliant rollercoaster ride. X-Men for TV, sort of.
Other Star Trek: Have you only watched DS9? I've been a Trek fan for years and there's lots of good to be found in the other series (perhaps mostly TOS and TNG). If it's DS9's arcyness that you liked, maybe ENT's third and possibly fourth seasons will grab your interest.
24: I'll throw this out first - arc-based highly-intense action. Mostly. The writing in general varied in quality somewhat over the years. Season 1 was the best, 2-3 were good, 4 dropped the ball somewhat, 5 redeemed itself somewhat, 6 crapped all over all the hard work done in 5. At it's best - densely plotted with good character moments and a lot of tension.
The Wire: Everyone should watch The Wire, easily the highest quality programme; the most pleasurable viewing experience I've had in such a long time. It has so many characters from so many different perspectives of Baltimore's drug trade that it's almost overwhelming to watch at first. It treats the subject with such honesty, and makes the characters so believeable and interesting that I was completely hooked after a few episodes.
Oz: Gripping prison drama, slightly OTT as the series went on but great characters make it worthwile.
Six Feet Under: Hilarious and heart-breaking at various points throughout the series, or even just thoughout an episode. The first two seasons were excellent, the rest was also pretty good - see Michael C. Hall in the role that made him. Also, best series-ending episode ever IMO.
Dexter: Fantastic show, excellent writing. Was getting into this when I was getting into The Wire so had lots of fun those months.
See Michael C. Hall in the role that made him fifty times what he was in Six Feet Under. A serial-killer that we sympathise with is an excellent concept in itself, but the show is more than that. Season 2 outdid season 1 somehow, can't wait for season 3.
The Sopranos/The Shield: Very new to me so can't comment on them properly, but very, very good so far.
I'll shut up now, I watch far too much TV.
And there's still stuff I haven't seen yet that people mention in this thread that make me feel better about all of the reality prime time shite that we get on mainstream TV.