TNG is my favorite (I grew up with it), followed by The Original Series. Basically, I think the quality of writing and the characters set TNG apart.
Picard, Data, and LaForge are my favorite characters. Guests? Guinan, Q, and Barclay were all great.
I think TNG evolved well as it went on; the other series seemed to stay relatively status quo, like DS9 and VOY.