For me it's a three way tie...
~Tremors: The Series~
Any fan of the movies absolutely had to have loved this series. Unfortunately it was cancelled so that a different, but much more advertised show could take its place. Said show was cancelled as well.
~Invader Zim~
Take all the reasons that people have mentioned so far and cram them together. There you'll find the reasons why I have Zim on my list.
~Blade: The Series~
I'll admit that the first half of the season sucked horribly but towards the end it became one of my favorite television shows ever. Sadly, the bad episodes at the beginning turned off quite a few fans and the show didn't get enough ratings for a second season.