Penny

Reason 1: It is original. I mean, the fact of choosing nerds for the main characthers is really great. It´s funny watch how this geeks are trying so hardly to be normal.
Reason 2: Penny´s characther. She is just a beautifull girl who change, in some ways the life of this geeks. I mean, thanks to her, Raj discovered that he can talk to women when his drunk; Howard is always flirting with her, Leonar fall in Любовь with her and Sheldon has make a lady friend who can stand by him.
Reason 3: the situations. The problem who this guys (and girl) must resolved are so funny. I mean, the akward conversartions...