Iron Man has a huge advantage with his flight abilities, very powerful energy projection, technology, intelligence...he can technically lift more weight too. In the old old days...when FF and the Avengers hadnt been around too long, Thing would have beaten him just because he was a more central character...back then he beat a lot of people he shouldnt have, while guys like Thor and Iron Man were really portrayed as weak since they were part of a bigger team. Then in the seventies i think they had a fight where Iron Man won. When the Thing was upgraded in the eighties I think he would have beaten Iron man even though Im not sure if they actually fought. Recently, while Thing has been portrayed as more of a 100 ton class character, Thing and Iron Man have bumped a couple times. One time when thing neutralized the whole Avengers...and then another time when Iron Man's armor was malfunctioning and trying to destroy the Thing and Mr Fantastic on autopilot. It goes without saying that Thing looked a little better than Iron Man since he took his whole team...and when the armor was malfunctioning Thing used his superior fighting skill, he actually said it was a Judo move that he knew Iron Man would not expect from someone with a reputation of a brawler, and neutralized Iron Man long enough for Reed Richards to disarm the armor. Thing also appeared to be stronger since Iron Man could not break out of his grip.
This is just another one of those cases where Thing looks like he should lose on paper but he still ends up winning...I give it to Thing in the categories of strength, toughness, and fighting ability...but Iron Man has the brains, technology, energy projection, and flight. In a one on one fight, out in the open, Iron Man should win, but I think Thing is too resourceful and tough to just count out.