Okay, I'm a DC fan primarily, so saying this is a big deal, but Iron Man is definitely the best superhero origin film since Batman Begins.
As for which is better, Begins or Iron Man, I'll call it a tie (as while Iron Man is the first appearance of the character on the big screen, Nolan had to ressurect Batman after Schumacher killed him - so maybe Iron Man is a slightly better film, but I think Begins had more ground to cover by totally reinventing a character that virtually everyone had gone off).
I loved, loved, loved MOS too but it hit a few off notes, whereas Iron Man was pure magic. I've never really liked the character that much (and hated the sequels, but mostly because they were such a disappointment after the first movie).
John Favreau should have hung up his director's spurs then and there, and be remembered as the guy who brought Iron Man to the screen and made him the coolest superhero around. Sadly, the subsequent films never built on
what was started in IM (even Avengers did **** all to develop the character, so after 4 films he's still just RDJ doing the same routine, the development in IM 3 was so haphazard it was off-putting -whereas the hostage situation in the first film was brilliant.
So despite my allegiance to DC, got to say that Iron Man is one of the great superhero origin films of all time. (Superman the movie was probably the best, but mostly because until then, nobody knew that superheroes could make it on the big screen).