probably supergirl seeing shes as-or stronger than TheThing as-or more durable as TheThing as-or more experienced as The Thing....plus she can fly and all that other crap DC considers cool.
even with his upgrade...shes probably on par with his strength.
I haven't seen her since the Superman/Batman storyline. Where'd she go. She should join the Teen Titans. I'd like to see a catfight between her and Wonder-Girl over Superboy.
The THING no question in a ROCK slide! Once he got past that not wanting to hit a girl thing. She would have to ROCK him a couple times, and then he'd realize that there is a super in front of her being a girl and he would then put a quick end to it. And ROCK her world!
Sorry but the Thing has absolutely no chance of winning. Superspeed, heat vision, freeze breath, flight, superior durability and superstrength, etc. The Thing would put up a good fight though