Hmm, what games do you have and enjoy already? That might give us some clues about what else you might like.
I don't know your library. But I am extremely partial to RPGs. It really depends on what you are looking for. Here's a short list of some of my favorites or just those that I plain recommend if you haven't played them/want them. -Dragon Age: Origins and Dragon Age 2. (Don't recommend Inquisition on it because it has it's hitches on the older console series) -Mass Effect trilogy. -Portal and Portal 2. -The Elder Scrolls Oblivion and Skyrim. -Minecraft -Terrarria Of course there are others, but I think that is a pretty solid list to start off with. I personally have all of those in my library. I suppose you could also go with the Halo series, although I myself am not a fan of them. All in what you prefer! Good luck building your new collection!
I'm a huge gamer and RPG fan as well, so my recommendations would be: -Fallout 3 -Fallout: New Vegas -BioShock series -Dishonored -Assassin's Creed series -Tomb Raider And also everything else recommended in this thread. I hope you find something you like!!