I agree.Depends what character you're drawn to most.
The early ones where they are initially designing the look of the characters offer the most variety.
The first Captain America, Iron Man and Thor (even Avengers, to an extent) have lots of great concepts where they're still trying to establish the looks of the characters. Guardians of the Galaxy, too.
The books for the sequels have less new ideas, I think.
Only after Avengers:Age of Ultron....here in Japan, the translated the Art of Book is released on bookstore...
The book is a little bit expensive (nearly ¥7000) but i think it is worth spent..
I really love the various concept of Ultron by Ryan Meinerding and Charlie Wen...
It really is an art...
It is shame in Civil War, there is no Spider-Man concept art
I liked more of the Ultron concept arts than how he appeared in the movie.