My ranking of the Hitman games:
1. Contracts: The first one I played. Loved the dark atmosphere and huge levels like Beldingford Manor and Traditions of the Trade.
2. Blood Money: The accident system was a huge plus, and I liked the numerous big sandbox levels. Disliked the new save system that didn't keep saves after you shut it off.
3. Silent Assassin: Some missions were total rubbish (i.e. The Graveyard Shift, Hidden Valley), but others such as Invitation To A Party, Basement Killing, and The Jacuzzi Job make this a great game. Tough too. I must have played Tracking Hayamoto a dozen times before beating it.
4. Absolution: The only reason this is not #5 is because the Contracts mode is brilliant. Actually, it's the only thing that makes it feel like another Hitman title, with all the different "Hits" you can come up with. The actual game is such a let down: the broken disguise system, the removal of the integral map, the idiotic checkpoint system that when reloaded has re-spawned enemies (WTF?!?!?!), no in-game saves, way too many missions without actual "Hits," and way too many levels that are "go through this door", "go from point A to point B" and "run from the cops again." The missions Run For Your Life and Rosewood are absolute abominations to the series, and had me nearly pulling my hair out. Some have compared these missions to those in Manhunt, but I don't agree at all. I loved Manhunt, and in that game you are not punished for killing, and the killings are actually unique. I will not be playing through the actual story mode again, and this will be the only Hitman I play one time through. Without the Contracts mode, this title would not be replayable at all.
5. Codename 47: Hated this at first because it was so primitive, but ended up enjoying it for the missions that were remade in Contracts, especially Traditions of the Trade.