I couldn't pick my favorite, so here's the top two.
There are very few TV shows that can make me laugh out loud every single time I watch them. Community does this without fail. It is SO funny! I wish I could be with the characters on all of their crazy adventures.
What a fantastic show! I could watch it all day, and let's be honest, I have done that before. I love the intensity - every single episode keeps you on the edge of your seat. It's so interesting to see how they figure out all of their suspects...
If you haven't seen either of these shows - go rent/buy/netflix them NOW. You'll thank me later.