Any Smash game. Each of them has amazing depth. They are easy to learn, yet hard to master.

The Knight of the Old Republic games.
Reason: There are so many different paths you can walk, so many different ways to play the game, itès really fun to try them all out.
Morrowind.
Reason: It is an absolutely massive game that takes hundreds and hundres of hours to do absolutely everything in.
Chrono Trigger
Reason: Multiple endings make it fun to play through over and over to try and see them all.

I'd have to go with the Metal Gear Solid series, having to make several runs to get all the items and in the later ones for unique camos