My pick would be the first one.

Thank you, Sir.

This needed to be said. The first two were passable entertainment but the third improved in every conceivable way. Better action, better jokes, better written and more eventful with a better narrative. And I believe there is one simple reason for all this- Shane Black.

There seems to be a strange perception of the first film that it's some sort of classic. People look back on it fondly but I think they are remembering it as better than it actually was. It was a straightforward, unremarkable but broadly entertaining film that got by on Robert Downey Jr's new Kiss Kiss Bang Bang persona. It received passable reviews at the time and was liked by many but loved by few. Today it seems to be loved by many and I wonder if that may be because it's the film that kick started the MCU.

Yet the third one seems to get a fair bit of flack. Mainly from comic purists angry because of the whole Mandarin twist. From a comic fan's perspective I can understand the anger. From a cinema fan's perspective it makes it a more interesting film because it explores the issue of propaganda, giving the film a slightly deeper flavour than the previous two. And ask yourself this: were the villains in the first two really that great?

The few action scenes in the first two all lasted about 5 seconds long. The set pieces here are well constructed with a narrative flow and most importantly: they last a decent amount of time, allowing the intensity to ratchet up. This is because Black knows action and Jon Favreau does not. Even if we concede that the Mandarin twist is an epic fail there's still plenty to enjoy throughout. The film is peppered throughout with inventive sequences and amusing little narrative twists. An example being the skydiving rescue followed by the reveal that Stark isn't even in the suit.

None of them are bad films but this was the only one I felt truly satisfied by.

@glomps and nibbles said:

Just like your love for the 3rd one , i love The Incredible Hulk. I find it stunning. Remarkable in my opinion. But i have hardly observed people appreciate it or even talk about it. And i actually find this version of Hulk better. May be Edward was not appreciated by the studios but i love The Incredible Hulk.

Right now, the Hulk from 'Avengers Version' is widely loved and praised. But the Edward Norton version nails it for me.

Hulk and Abomination fighting right in the end was jaw dropping.

Ed Norton was very good in The Incredible Hulk, and added some nuance to both sides of the Banner character, as does Mark Ruffalo. My main beef with the film was The Hulk himself, in terms of animation. On the big screen, he simply didn't cut it for me, and I wondered if and hoped that Marvel would improve their CGI enough to make Hulk look basically flawless. With The Avengers, to borrow your term, they nailed it.

3 was my fav aswell. I prefer comicbook movies where the hero is established from the get go. 2 was kinda crappy storywise.