I'm enjoying SG:Universe, it's very slow-paced but pretty atmospheric.
Had The Wire very heavily recommended.
It's an aquired taste, but (as a sci-fi nerd) Farscape is one of the best sci-fi shows ever (apart from Firefly). Well, well worth getting over any preconceptions and giving a chance, it's a superb mix of dark and witty.
New one that I've yet to watch but is supposed to be ace is Breaking Bad.
If you're a geek, Chuck and The Big Bang Theory are dripping with in-jokes. TBBT is like a US version of the IT Crowd, only considerably funnier.
Dexter is supposed to be ace, again I've not got round to watching it yet.
The first series of Heroes was great, then it kinda didn't know what to do with itself.