Personally I'd have to go with the Vegas mission, the music was really great and climbing around a hotel in Vegas from window to window was truly bad-ass. Really liked the interaction and the keycard system, and figuring out how to get into the different hotel rooms. 
 
I have to say I also loved the Mardi-Gras and the rehab clinic. As a bonus I'm going to have to say my least favourite was the Mississippi river level, it was definitely the least interesting.

I'll agree that the Mississippi was the worst. It's really tough to pick a favourite, I loved A New Life for its simplicity (in that it was a small level and the situation was very clear) House of Cards for its complexity.

I liked all of them worst was probably the Boat level, the first level and the wedding level was quite boring.

• Curtains Down
• Flatline
• A New Life

House Of Cards probably. And yes, Death on the Mississippi is a piece of shit.

• A New Lifes
  
• Curtains Down
• House of Cards

• A New Life 
• Flatline
• Amendment XXV

All the levels, they were all equally great imo.

The one where you can dress as Santa.

Tough pick between House of Cards and Heaven/Hell. I'd have to give the edge to Heaven/Hell because the atmosphere in those levels was so awesome and somewhat creepy at the same time.

i couldnt get past the 4th level cuz the game just felt bad and broken to me.. AI sucked ass

Easily Flatline. 
Something about being in a rehab facility makes me feel so at home.

The one with the hell-party.

Suburbs. I played that mission over like 30 times. 

Ugh, if I could remember the names of the missions it would have been easier to say the ones I really liked. I think it's A New Life it's called, my favorite mission. The one with the guy in the house. Kinda small map but still lots of fun exploration. 
 
Woaa! Wait! Also The Murder Of Crows is a damn kewl level.

The Murder Of Crows   is a brilliant level, i dont know how many times i tried killing all the people at the parade :) 

Vegas mission was really cool but far to short. You can do it all in the elevators. I would have to go for the Rehab level. Loved it so much! Worst would be Murder of the Crows. Pain in the ass!

I loved both missions in Las Vegas. The Casino and the rave. I also loved the Murder of the Crows, get infinite ammo and kill everyone there. I also liked the opera level. Overall I like most of the missions and consider it the most complete Hitman game so far.