Garth Marenghi's Darkplace. I want to see the other 44 episodes!
I'll pick 15 Storeys High as well. I think if they showed it now at a reasonable time with a little bit of promotion (as apposed to the zero promotion they did first time round) it would garner respectable viewing figures and much critical acclaim.
Would have loved more Father Ted (I am aware why this is not possible).
On the American side of things I would have liked to have seen a full 22 episode series of Police Squad (does it count as a sitcom? I'm not sure).