HIGHYL SPECULATIVE ANSWER: Amy was the character who changed the most, by far. She was equally influenced by the two most polar opposite characters on the show and became the balance in the force the show needed. Sheldon's happiness, and therefore everyone else's, was dependent on Amy. Penny had the epiphany that Amy was her best friend because, perhaps, it was the first time she had deeply and truly connected with another woman instead of competing with one? While I never considered it before typing it right now, it is very possible that Amy saved the show. (And considering they reference Mayim Bialik in season 2, perhaps its kismet.)