I'm assuming that by best, you mean the person that is outgoing, confident, and is strong in the face of adversity. Really, I think it is tied between Mercedes, Kurt, and Tina. They are all their own person. They don't try to copy anyone, and are true to themselves. THAT is the best personality anyone could have. :)
The best personality has got to be Sue Sylvester!!! I love her to bits and Glee would be not watched by anyone if it wasn't for her! She has a comedic and angry side that viewers cannot get enough of!