Even after all these years the answer is and probably always will be Xander. I first noticed him during The Pack. In fact I can tell you the exact moment I noticed him:
This is a guy who's normally all sweet and goofy and I love him for that but to get the teenager I was at the time to pay attention he had to get possessed by a hyena and act like a douchebag for a whole episode. Though I will have to say - eating a live pig ain't hot.
Favourite female character
Do I have to pick just one? Really? Can I not have three? No? OK then...
Excuse me, my brain always stops functioning after looking at the hotness that is Ms Naya Rivera many times over. Santana is played by someone who could be genuinely called "a triple threat" unlike some other members of the cast I could think of. Naya can sing, she can dance and goddamn she can act. She makes Santana the best thing about Glee. Even back in the dark days when she got barely any scenes she brought out the amazing-ness in Santana with her partner in crime Brittany and boy, did it pay off. Bit part to recurring character through to regular all in one season. She should be the lead of this show but instead Ryan, Ian and Brad would rather make it the Rachel, Kurt and Finn show.
(Oh btw, if I could have had three, the other two would have been Brittany and Willow, in case you needed it spelling out ;-))