AC II and AC Brotherhood were the best for the core mechanics of the game. They had the best character, Ezio, the best environments and the most exciting armour and collectibles. Revelations was great too.

AC IV:Black Flag was interesting but the ships don't make up for the fact that it lacks the excitement of Constantinople, Rome or Florence.

For me its Assassins Creed 2.

Assassins Creed was one of the first games I bought for the PS3, and despite its flaws, I still had a ton of fun with it. AC2 was a huge improvement from the original, refined combat to mission design to the open world, I ended up getting the platinum. I even printed out a map with all the feather locations lol. From there I felt like the AC series was getting milked and the formula didn't change (aside from a tacked on multiplayer mode) which made me lose interest in the franchise as a whole. Played all of them up to AC3 but none of them gave me the enjoyment that AC + AC2 did.

It's a toss up between 2 and 4. Two still had the great logic puzzles which Brotherhood ditched. Sailing around the open world of Black Flag was amazing. I picked Black Flag for the water.

Poor Rogue, only game without a single vote.
I never actually played it, as while I enjoyed AC4, I was also a little burned out on the series by the end of Gen 7.