In somewhat of an order...
I also have to vote for Red Dwarf!
Hooray! According to Robert Llywellyn's Youtube blog We are getting 4 (well, 3 and one behind-the-scenes) new special episodes that will finally wrap things up a bit.
Futurama. Yes, I'm listing a cartoon :) But it's a good one!
LOST: If only for the first season, it's a bit more bumpy afterwards but still good.
Also: Stargate-SG1.
Not the best sci-fi out threre, but it is rather enjoyable and usually not that serious to make it fun.
Heroes, as others have mentioned. Pity the effects of the writers strike, season 2 was nothing like season 1.
---
Last night, I watched the Day Watch movie (Or Night Watch 2, if you prefer). Various notes around the net talk about creating a tv series/miniseries off the series of novels. I'll be waiting for that, even if it is more Urban-Fantasy, but the SF&F genres are getting categorised as Speculative Fiction nowadays anyway :)

i just wish red dwarf would never end..

75% of my DVD collection and probably 500gb of the space on my HTPC is sci-fi so I'm not listing them all but here's the highlights:
Dr who
star trek (all series)
firefly / serenity
babylon 5 (the first series is fairly good but it gets progressively better)
earth final conflict (the first series is fairly good but it gets progressively worse)
andromeda (the first series is fairly good but it gets progressively worse)
dr who
Robotech
Macross Zero
Macross Frontier
Dr who
V / V The Final Battle
star wars
dr who
original battlestar galactica (hate the new one)
dr who
Space: Above & Beyond (even though it's the only battlefield democracy I've ever seen)
dr who
the day the earth stood still (The REAL one, with actors rather than keanu reeves)
dr who
transformers
transformers cybertron
dr who
odeyssey 5
dr who
torchwood (series 2 was actually fairly good, they kept it in their pants for the most part)
the sarah jane adventures (slow start but some pretty good episodes in there)
dr who
dr who
dr who
and did I mention dr who?
I think that adding red dwarf as sci-fi is a bit of a stretch, but if we're including sci-fi themed comedy then add futurama & 3rd rock from the sun

I have to agree that Planetes is pretty amazing, and sadly enough incredibly underrated.

In no particular order, here are some of the sci-fi shows I have enjoyed, cartoons included.
Firefly
Farscape
Stargate SG1
Stargate Atlantis
Sliders
Fullmetal Alchemist
Ghost in the Shell: Stand Alone Complex

I always liked first three seasons of sliders. So fun to watch, enjoyable characters.
Big fan of voyager. not sure why. don't like any other star trek (few ep of deep space nine were alright)
firefly was great. shame it ended. or maybe it is a good thing? so it couldn't become shit like most of the tv series these days.

Red Dwarf is as much of a stretch as Star Trek. :(
3rd Rock from the Sun is alright, I mean, I don't think I'd ever ask to watch it if there was a full selection, but say if I was forced to pick between that and reruns of Urkel, I'd... I'd go for Urkel..
I mean "Family Matters"

I'm watching 3rd Rock from the Sun right now on TV Land and I cannot stop laughing!

erm. 3rd rock from the sun is not a s-f series you know.
I enjoyed Farscape. Seen only first season and although science was a minor part of the series it was fun to watch it.

Oh I know, I could see how many people don't like it, it totally does it for me though.

My favorites are
BSG, Red Dwarf, Star Trek TNG, Twilight Zone, Quantum Leap
I tried watching a few episodes of Firefly but hated it. I know I'm in the minority on that one, though.

I've been on a Quantum Leap-athon for a few days now, thanks netflix!

I have been watching Stargate from the begining to the end which i must admit is a really good show and dosent take itself too seriously.
BSG is also fantastic although the split in series for 4 and 5 is a pain the wait is too long sigh.
And the best british Sc-fi i watch has to be Dr Who since coming back has been one of the best sc-fi series in years.

I've been considering doing that myself for a while now.. Right now my netflix queue is on DS9.