Avengers : Age of Ultron is very underrated but it is a lot of fun, and is one of the best movies of Marvel because:

I) It is the first movie where Hulkbuster was shown.

II)Scarlet Witch and Quicksilver were introduced in the movie.

III)It was the first time when an infinity stone was a part of body of a robot - Vision

All these features make Age of Ultron Marvel-lous.

It is great