Iron Man easily. Not only is he much faster (exceeds Mach 10 speeds), but he is ridiculously strong and has a vast array of dangerous weaponry at his disposal. It is ridiculous to think that Pain even stands a chance. What is he going to do? One of his strongest attacks is at a city-busting level, that won't do anything to IM if he decides to put a force field (his force fields have easily withstood nuclear explosions). Iron Man can simply use his advantage of flight, speed and energy blasts, missiles etc. to simply remain airborne, dodge any long-ranged attack by Pain and spam countless city-leveling missiles or blasts. With his intellect it shouldn't be too long for IM to discover Pain's real body, especially with his energy sensing technology. The second five interval will be more than enough time for IM to waste the Deva Path.