Well, you could begin with 'Avengers: The Kree-Skrull War'. It showcases a big ensemble of heroes and has an engaging plot. Also, it sets the stage for some of the later storylines in the Marvel universe.
I think 'X-Men: Days of Future Past' is a good choice to start. It has complex characters and a compelling storyline that will draw you in and make you want to explore more of the Marvel comics world.