While Mass Effect 1 has, in my opinion, a better and more epic main story, Mass Effect 2 has a wide amount of high quality side content.

This game has an impressive cast of companions to recruit, get to know and complete missions with. Each companion has a unique loyalty mission that is required to get the best ending. But many of these missions are secondary, so you could miss on a lot of good content.

Overall, this game is a worthy sequel, boasting visibly higher production quality (animations are less stiff and cutscenes are much more frequent and cinematic) as well as delivering interesting side content the first game was super lacking in.