First of all, Titus. Seriously, they had 3 seasons of pure awesomeness and they cancel it? What the hell? Some might say that it's better it got cancelled before it started to decline, but this showed no signs of that what so ever. It was just brilliant from start to finish.
In second place: Futurama. Why this got cancelled I'll never understand. Even so, it was nice to see it coming back to life with the movies that were really good (except Bender's Game. That one sucked). I just hope that the rumors are true and that it will come back on the air.
Third place: Nowhere Man. Great show that only aired one season in 95-96. It was planned for a second season, but got axed. Which sucks, because the ending of season 1 was a real cliffhanger.
And I kinda have to mention Family Guy. Sure, it came back, but it was nowhere near as good as the first three seasons. Seth McFarlane should be ashamed of what Family Guy has turned into...