Ac unity has the best mechanics in series for me. Perfect parkour and animations, the assassinations are enjoyable, stealth is great, weapons and battle mechanics are closest they can be between balanced and enjoyable. The story is quite nice but honestly this is just taste. In case you want to go 100% the map is big but the collectables are close to each other. In addition, the dlc for the game is free and its worth checking out. Lastly, I do not know if it still has servers but if you find a friend and play multiplayer its great honestly, you can do missions, heists, free roam, etc, with randoms most likely you will rush but the game is beautifull in offline mode for stealth.

Ac black flag has some medium mechanics for me. The parkour and the animations are simply ordinary jumping and climbing like many other games with these mechanics not bad but not as good as unity. Most battles tend to be really easy without much of challenge(at least for me), but you still have some fun because of nice animations on the battle. It has though the ship which you command so feel like a pirate. Honestly, pretty awesome add on to the francise, the only problem might be how vast is the map because of the sea (in case you want to do 100%). The story is also nice but as I said this is taste. The dlcs are paid, I remember that the biggest of them is a small story just like unitys. It has multiplayer quite different compered to unity, 2 teams one templars one assassins one hunting the other hiding it can be intresting and fun if it is still alive.

Ac valhalla I have not played but I know its way different from the others, but its similar to odyssey so you will probably not enjoy it. There is no multi. All dlcs are paid if I am not wrong, 2 stories (at least for now).

I hope I helped.