In the comic books, Tony Stark is a smarmy, not exactly pleasant guy. To that end, I think the writer and RDJ did a bang up job.
Back to the OT, I thought they were both great - as they both achieved what they set out to do perfectly. Of course, what they were trying to do were pretty radically different things. The one I can reasonably objectively call a "better" film is Dark Knight. The one I know I'll watch repeated times, Iron Man.
So... Ah... I don't know. I'll plump for Iron Man, purely for the unabashed fun factor.