My favorite is a tie between 1 and Unity.
But the best one is probably II.
The whole franchise from 1 to 3 really needs you to play all the games.
4, Unity, Syndicate, Origins and Odyssey don't, you can pick any of those when you feel like and its all good.
Im doing my best to not spoil you, if you play any game from 1-2-bro-rev-3 on its own without playing the previous one, you will miss 70% of the experience.
You can buy 3 and then pirate 1,2 brotherhood and revelations, but if you just buy 3 and play 3 it will quite literally be like watching endgame as your first MCU movie without knowing shit about the universe.