It depends on what you like really.
If you own Civ IV, Civ V is probably not worth the buy. But otherwise it's a solid 4x game if you also get the God's and Kings expansion.
TL2 is a good Diablo clone. Not as tight in the design as D3 but with more options. The story is not spectacular and the graphical design a little whimsical, but the gameplay is fun and it's a better single player game than D3 due to selectable difficulty levels.
Witcher 2 is a great action RPG with emphasis on story. Good characters and an immersive setting. It features a few different story branches but is otherwise mostly linear. Control is a little sluggish and some of the boss fights could do without quick time events but great game overall.
I haven't tried Mark of the Ninja, but I am tempted.
I bought "Mirror's Edge" myself on Steam sale. If you haven't tried it, it's certainly worth consideration. If you are familiar with the AC serie, Mirror's Edge is a game solely about the same type of parkour mechanics, running on roofs and walls, jumping up and down stuff. Very good campaign, unique graphical style, but probably not much replayability for most people.
http://store.steampowered.com/app/17410/