Have to disagree... After Bryan Fuller left mid-first season, the show started a dive, which the second season did not avert. Season 1 holds up pretty well, but Season 2 just doesn't hold a candle to Season 1. And do not, I repeat, DO NOT, watch the direct to DVD follow-up movie.
While I enjoy his other series, including Pushing Daisies, I would say Wonderfalls fits the bill for this thread. Cancelled part way through its first season, but allowed to finish filming and wrap up, I feel it benefitted from cancellation in the sense that I don't know how sustainable it would've been. As a 14 episode run, it's damn near perfect.