I also happened to really like this movie and I agree it's underrated. I may even prefer it over Iron Man 1, though I still think Iron Man 1 is a better movie from an objective point of view. Out of the six Marvel films, I think Iron Man 1, The Incredible Hulk, and The Avengers are the movies to be considered as some of Marvel's greatest effort. They're fantastic IMO. Iron Man 2, Thor, and Captain America are good too but I felt they could've been a lot better.
Part of what hurts Iron Man 2, Thor, and Captain America is that they spend too much time setting up for Avengers and bringing in other characters from the Marvel universe. It made the Avengers movie better and hyped it up very well but the solo films themselves suffered.
If I were to rank the Marvel movies from an objective film point of view, I would rank them like this:
Great movies:
1. The Avengers
2. Iron Man
3. The Incredible Hulk
Good/Alright Movies:
1. Captain America: The First Avenger
2. Iron Man 2
3. Thor
If I were to rank the Marvel movies from an enjoyment and preference point of view, I would rank them like this:
Movies I enjoyed a lot:
1. The Avengers
2. The Incredible Hulk
3. Iron Man
Movies I thought were fun/decent:
1. Iron Man 2
2. Captain America: The First Avenger
3. Thor