Assassin's Creed: Brotherhood brought so much variety... The crossbow brought sniping kills to heavily armored guards, while maintaining stealth. I have to admit that I thoroughly enjoy keeping to a "no collateral damage" mantra, though it is hard to maintain the further you progress in the game. For the multiplayer, I love the way the Harlequin slithers over and around her target to deliver the killing blow... very reminiscent of Scarlett Johansson's hallway fight scene from Iron Man 2.
Metal Gear Solid series ingrained a deep need to live and let live. But from MGS2 and onward, I've taken to holding them up and putting a tranq dart right between their eyes after they've been shaken down.
In Mount & Blade: Warband, I have a Nord character that has become renowned for his prison breaks... many a purse has exchanged hands for his retrieval of many a nobleman's relative or loved one. In a city, he'll start with a Heavy Great Axe to cleave the first guard's head in twain, then switch to his shield and two stacks of Balanced Heavy Throwing Axes. The remaining soldiers will approach, raise their weapon (and lower their shield), and catch an axe blade in their skull from four paces away. If the prisoner is being held in a castle, then he carries 3 stacks of throwers and his shield, help reduce the number of archers from range, or at least gain their attention to keep them from pincushioning the escaped prisoner.
The Hitman series was very fun to try and make a clean kill and getaway in, but sometimes a good garroting or crossbow bolt to the head was needed to clear a path. Being able to gain entry into a compound then slowly take out each and every patrolling guard, leaving just the unaware mark, was a unique challenge in itself. I think I replayed the Italian villa mission from Hitman 2: Silent Assassin over a dozen times, each time with a new strategy or personal challenge.