The Hitman games follow a simple but brilliant premise: you are an assassin with one or more targets on a map full of security personnel, civilians and all sorts of places to go and things to use to accomplish your objectives. You can assume the identity of others by rendering them unconscious (or dead) and stealing their clothes. Highly interactive and open ended (especially the fourth title, Blood Money) this approach creates a massive amount of re-playability, and considering we're talking about four games, that makes this a serious bargain.
Although there are certainly some missions where there is an obvious way to complete your objectives, for the most part there are plenty of different opportunities to get close to your target. You could dress as one of his / her bodyguards, you could sneak in as a delivery boy or a tourist or one of many other different guises including pool cleaners, rednecks, waiters and loads more. Once next to your victim, you can utilize the environment to fake an accident (such as food poisoning, a pyrotechnics accident or a weak chandelier fixing to name a few). Of course, a few bullets from a silenced pistol do the job just as well. In fact, you are welcome to blast your way through the levels with a machine gun if your attention span should wane! The games are full of dark humor, with plenty of opportunities for creating your own (ever gone on a killing spree dressed in a clown suit before?) and the gameplay is unique and entertaining.
Obviously, the older games aren't exactly polished in modern terms but are still highly enjoyable nonetheless. Probably my personal favorite game series of all time.