I have to say Claire. Playing her on the second level is the biggest challenge the game has to offer (not including the bonus games, of course). Plus, I have a lot more respect for Claire. While Leon was constantly chasing Ada with his hard on and saying "let's split up, Claire!", something about him seemed petty. Claire was a regular civilian, and Ada obviously had skills, and a certain degree of training, so why send Claire out on her own and stick with her? It was always about him trying to get the girl. In fact, in Leon B, I recall "Why doesn't anyone ever listen to me?!", being his most used line. I could just see him turning his head and saying, "GOD!", much like Nepolian Dynamite.
In the meantime, Claire has no training, but is taking on the responsibility of protecting little 12 year old Sherry. She's blasting through hordes of zombies to save the life of a little girl. She is one of the most selfless characters in the entire series, and she is always helping somebody. She's a hero in the brightest of lights, and emotionally and mentally stronger than most the guys. She will always have my undying respect. Plus, I just take more interest in her story.
Don't get me wrong, Leon fans, I think Leon's great. He is kick ass, and has a certain feel about him that makes you feel like a bad ass every time you decapitate a zombie with your trusty boom stick. His concerns, however, are much more petty than Claire's. By the way, when in hell are they going to bring Sherry back into the mix??