Torchwood: Not cancelled, just in a dormant state I hope.
The last two seasons were different and brilliant in their own way.
I don't know if this series could continue, they'd have to introduce new characters to work with the existing Torchwood crew.
They also have a bad habit of killing them off though, grrrr. They should have kept that blonde FBI staffer instead of that Rex character.
Merlin, even though I knew where it was going they didn't have to go there. Plus, that last season was terrible and I think Morgana was overused, bleh. Actually on second though, good riddance. Where in the world is Bradley James now?
Being Human UK, I think the series could have gone at least two more seasons with the new cast.
I enjoyed them immensely in their final season, plus the vamp character was hot and frequently shirtless. I wish we'd seen more backsides though.
Stargate Atlantis: The series I think was heading down a good path with what I thought was setting up new storylines only it got canned so what had been set up had to be scuttled away in favour of an endgame scenario which was rather un-convincing. I wish they hadn't ended the series the way they had ended it. They should have just kept it going where the characters were still in the Pegasus galaxy fighting the good fight... in the event that they would ever pick it up later in the future. Although I know that is now an improbability. The original writers/producers are planning a reboot and scraping everything about the series.
Star Trek Enterprise: I enjoyed the last season more so than the previous 3 seasons. Had it been in the hands of Manny Coto and the Reeves Stevenses' it probably would have seen a full 7 years run. The problem I had with the series is that Scott Bakula was a terrible choice for the Captain, I wish they'd had gone a little more wild with their choices. He was too nasally in voice and "EEEEEEEEEE". Still, plenty of shirtlessness on the series. BTW killing Tripp was a douche move!
The Sarah Connor Chronicles: FOX had to go and order more episodes than necessary, it ended up feeling like the second season was filled with padding, filler episodes. The way it concluded left something to explored... I would hope but this is highly unlikely.
Honourable Mentions:
Robin Hood BBC: I mainly enjoyed watching this series for the menfolk, the series itself was too kitsch and family friendly.
Had it been a more mature show with some darker occult elements it would have succeeded.
I hope some one at the BEEB will explore Robin Hood again and bring in those elements. I remember watching the original BBC Robin Hood, the one from the 70s or 80s. The one with Sean Connery's son, although he didn't take over until the second year I think.
*Hey Russel T. Davies, why not do a gay Robin Hood series. One where Robin and Sir. Guy are enemies/lovers. There'd be lots of nudity and general nakedness going on, not to mention big sweeping stories.
The X-Files: So admittedly, the series went down hill after Duchovny left.
I just wish that they had introduced Doggett and Reyes around season 6 so the audience would have accepted them stepping in and taking over.
Additionally, killing the lone gunmen was a dumb move as well. The series lives on in comic form.
I think this one has the possibility of coming back as a series, its been many years since it wrapped up. A series focused on Scully and Mulders child could be interesting, perhaps he gets abducted by some sinister force/group/etc. for nefarious means.
This leads me into...
Farscape: I miss seeing Ben Browder in tight leather pants. I miss the insane wacky storylines. I miss that puppet with the bushy moustache. Actually, I hear that this series might be coming back in a pilot movie. It'll feature Aryn and John's son in some capacity. Ben hasn't been in much of anything in awhile which is a damn shame.