I'm a big fan of all the Lego games, especially the Star Wars Complete Saga. Obviously it's not as difficult as many games are, but I'm not a pro-gamer, so that doesn't really bother me. There are lots of problem-solving type scenarios that are fun to figure out and re-explore at a later date with the new characters you've unlocked.
This was probably the first game that my son and I played together and it was a blast doing so. It was the first Lego game we got - now we have all of them (I think), including Harry Potter, Indiana Jones, LOTR, Marvel, Batman, etc...