Quite possibly my favorite of all the Hitman games, including absolution. While the story is still a main point of the game each of the missions can be played individually for fun.
When people talk about Hitman challenges, this is the game they mean. Once you have finished the main story line there is still incredible replay value in picking your own target and trying to make it look like an accident or going silent assassin on them.
This is one game that everyone should play, although the graphics may not be up to scratch with today, the gameplay massively surpasses anything currently available.