Assassin's Creed: Syndicate
I probably have the fondest memories of AC2 and Brotherhood, but it's undeniable that Syndicate builds upon what those games did well, expanding in areas that felt thin before, while inversely streamlining others that were bloated or unnecessary. It's definitely the most "complete" Assassin's Creed experience around, I think. Also, it has barely any current era stuff in it, which is fine with me after what happened in AC3.
Assassin's Creed IV: Black Flag
I'm so glad someone in a Ubisoft meeting said, "Hey, ACIII sucked, but everyone really liked the boats. Let's just make a whole game of that!". The Assassin's Creed parts of Assassin's Creed IV are about as formulaic and expected as it gets (there are STILL eavesdropping missions?), but the part where you get to be a badass pirate singing shanties with your mates, plundering warships, and taking over strongholds, is so goddamn fun that I didn't even care.
Assassin's Creed: Brotherhood
Brotherhood takes everything AC2 did to make the franchise legitimate and polished the hell out of it. Other than Syndicate, this is the ideal "core" Assassin's Creed game with nothing but a big ass city to climb around and dudes to assassinate. Plus, it has Ezio. Everyone loves Ezio.
Assassin's Creed 2
The beginning of Ezio's trilogy is what proved that Assassin's Creed could deliver on the promises it made in the original game. I still remember being blown away when I first donned the Assassin's gear and the game truly opened up. I remember thinking, "Holy shit. They actually did it. They delivered".
Assassin's Creed
The original Assassin's Creed was a clunky game, but an endearing one. I actually didn't get to play it until it released on PC, which I believe cut out a lot of the filler that people criticized in the console releases, so my memories of the game are probably a little different from many others'. It was an amazing proof of concept that AC2 would eventually deliver on. One of those games that made me more excited for the future possibilities it brought to the table, than the actual game itself.
Assassin's Creed: Revelations
Speaking of filler, Revelations definitely felt like such to me. I enjoyed seeing Ezio one last time, and it was cathartic seeing his arc come to a close, but everything outside of that just never really clicked for me. The environments weren't terribly interesting or enjoyable to move around in, the yearly gimmick in the form of bombs was mostly useless and I only ever crafted them when the story demanded it, and the modern day storyline was so weird and disjointed, it could of killed my enthusiasm for seeing what was up with Desmond leading up to AC3. Plus, he looked like a mix of Nicolas Cage and Adam Sandler. What was up with that?
Assassin's Creed 3
I can't say too much about AC3, because I never finished it. However, I can say that the reason I never finished it was because of how little I enjoyed it. Everything was just so... bland. The story, characters, setting, exploration in the frontier... none of it clicked. Granted, I didn't get far; I think I got through the first tutorial describing how to set traps when hunting in the frontier, and threw up my hands saying, "Nope. Can't do it". I did like commandeering ships, and Ubi making a whole game about that guarantees I have no need or desire to ever go back to AC3.
Never played Unity or Rogue, so I can't comment on them.