I expect most everyone wants to say Firefly, and then decide that its so true, its just Cliche. But yes, Firefly.
Also, Dollhouse. Yes, it started a tad slow, and the very end of season 1 was a bit odd. However, the mid to end of season 1 was pretty great, and the current (and final) season is absolutely amazing. Far denser television then you can expect to have mainstream appeal. I understand why it is canceled. It's just so disappointing.
Also, this girl http://l.yimg.com/l/tv/us/img/site/32/22/0000053222_20081110111439.jpg
I'm normally not swayed by a pretty face, but...damn... ::shallow moment over.::