Well it's hard to say. I haven't played the most recent one, unfortunately, because I don't have, or ever planning on getting, a 360.
But again, it's hard to say.
AC3 was what got me into the series and had a great overall design. Very technological and futuristic, I loved it. Going up into space in the blackbird and chasing that stealth bomber into the underground city was pretty damn awesome.
AC4 had a good story and the levels were fun. Music was great, as usual. I loved the hand-drawn cutscenes, they complemented the story well and helped create a nice vintage-esque theme. If I had a complaint it would be the lack of input from the main character, but that's part of the story I guess.
AC5 is probably my favourite. The new planes, squad leader commands, and 3D cutscenes were great. The story was a better written than the previous iterations, and the death of Chopper was immersing, there was a lot of emotion there that you wouldn't expect in a flight sim. This one had the best soundtrack so far, too. The track 'The Unsung War', which plays during the final level, is pretty enthralling. It was also pretty fun to go back and play as Mobius in the Arcade mode.
Then there's AC:Zero. I loved the new multiple path system. The game was less story driven and instead placed an emphasis on the philosophical meaning behind the choices that the character took on each mission. The whole 'jet fighter western' thing it had going on was pretty awesome, too, what with the Spanish duelling music and the Round Table Duels. I have to say that the final confrontation with Pixie was just great.
Again, it's hard to say which is my favourite, as this is my favourite series and I love each game individually and on the whole. I guess though that if I had to pick only one to play again it would be The Unsung War. It's probably the most fulfilling AC game that I've played, and has the best combination of story, gameplay, choice, sound and visuals.
so yeah AC5