Legendary: Dark City
Buy this firstA BGG blog ranks it first of all Legendary expansions as the set that completes the base game. One poster started with the core set, Dark City and Guardians of the Galaxy.
Needs: Just the base game
Adds: Seventeen new Heroes such as Cable, Daredevil, Professor X and Blade, plus Masterminds like Apocalypse, Mr. Sinister and Kingpin. It has 350 new cards and Special Bystanders.