Depends on what you're looking for. Being an action junkie, I was never a fan of just about any of the Start Trek series, myself.
I reckon Firefly needs very little introduction, but I don't know if you've actually seen it yet. The most undeservedly cancelled sci-fi show I've ever seen.
Of the more recent series, I thought Lost was good at first, but when all the mysteries of the first few episodes were still left unsolved a couple of seasons later, I was starting to get my fill. I can definitely recommend Heroes, though; the first season blew me away, and while the second hasn't been all that great (and with writer strikes, the next episodes have been postponed till September), it's a great, great show! Definitely recommended if you can at all swallow the "X-men principle".
On the humour side of things, I've found that I actually like "How I met your mother" very much; I feared at first the show would resemble "Friends", it's funnier, wittier, and much, much sharper. Not sure how high it rates on the Geek meter, though.