Hitman 2 - Basement Killing: It's first of the three missions at the Petronas Twin Towers in Kuala Lumpur, Malaysia. I've actually been to this location several times for work and pleasure. Of course the interiors at definitely more breath-taking than how the game looks, and no, there isn't muslim police in khaki shorts shooting at you at every opportunity there.
Hitman 3 - Hunt and Hunted: It's really exciting evading the load of SWAT teams and policemen, and then taking out the inspector on street level... when he's surrounded by countless SWAT team members and policemen. The most satisfying thing is escaping in the fireman's truck. It's like a big fuck-off to all these people trying to kill 47.
Hitman 4 - Flatline: Best use of accidental kills in my opinion. All three targets could be killed by accidents.
Hitman 5 - Rosewood: Hands-down the hardest level on the Professional difficulties. Obviously satisfying to get out alive in one take.
If it's gonna be just one choice, it'd be Hunt and Hunted, escaping in the fireman's truck.