It's a tough one between Odyssey and Black Flag. Black Flag is truly AC game I fell in love with. It's pretty much perfect in many ways. Sailing your ship with your crew made it a really personal experience even though you didn't know anyone on the ship but Adewale, and since then Ubisoft weren't able to capture this again. Story wise it is one of the best character arcs that AC has had (Ezio is a better arc, but he had 3 games to flesh that out).
On the other hand Odyssey is balls to the wall insane, which I wanted from AC for a long time. All those monsters and gods (in DLC) and Atlantis... It felt so good to actually finally see the side of AC that was only teased up until this point or sometimes wasn't even teased at all in some games, and that side didn't disappoint me. Sure, Odyssey doesn't have sea battles that are as fun as Black Flag, but it's on-land gameplay is much better in my opinion. I guess if I had to choose, it would be Black Flag, because not gonna lie, I almost cried at the very end of Edward's journey, while I didn't feel particular attachment to the character in Odyssey.
I would also have Revelations and Rogue as runner ups because I feel like both of those don't get enough love, even though emotionally Revelations is the strongest out of all AC games, and Rogue is the most unique, letting us see Templars as realistic humans, not just evil dudes (something I hope we'll get again sometime).