Future Perfect.
Best single player mode (story was hilarious). I loved flipping from time period to time period doing different stuff. Siege on the Scottish castle; the train chase & the haunted mansion stand out in my mind.
Same great multi-player as ever. Virus returns, along with a load of the great maps from TF2 and a load of fantastic new maps.
They fixed the aiming so it didn't snap back to the middle as much (at all?) compared to 1 & 2.
I will say that I think I preferred the multi-player maps in 2 and the single player did have a lot of memorable locations, but overall I'd say I had a lot more fun with Future Perfect.
The co-op campaign worked better in FP than it did in TS2 as well.