The only Assassin's Creed I ever played was the first one which bored me after a while because it was extremely repetitive. However I wouldn't mind trying other games in the series but there seems to be so fecking many so which ones are the best and which ones should be avoided?
Honestly, the new games and the old games are entirely different so ill give you a brief (actually quite long) summary:
Older games:
AC2 is a fantastic game and still holds up really well even now. Ubisoft really built on the success of the first game and fleshed out many of the systems. In my opinion, you have to play AC2 if you want a true sense of the AC franchise. AC Brotherhood is also an excellent game, honestly, the whole Ezio Trilogy is just great as he's such a charismatic protagonist. AC3 is decent, but many people found the protagonist a bit boring and I think that put people off, but it's a good game overall. Black Flag is the first game that moves away from the traditional formula and was a fun, unique game but I wouldn't recommend it because the newer games e.g. Odyssey offer naval combat with so much more in between).
Newer Games:
Personally I would just skip some of the middle order games, I.e Rogue, Unity, Syndicate. They aren't that bad but they just are a bit samey. So that brings us up to the newest games Origins and Odyssey. These games have both dramatically altered the original AC formula and in doing so might as well be different games, however, I don't necessarily think its a bad thing. Both Origins and Odyssey introduce more RPG elements alongside truly massive open worlds. They are both good games, however, if you are looking for something with the depth of the Witcher then don't bother as they remain typical Ubisoft games; big, great worlds but lacking that real soul.
Summary:
Personally, I would recommend everyone plays the Ezio trilogy even today as they are just classic games, and having played them again recently, they still hold up very well. Then I'd say pick up either Origins or Odyssey, probably not both. Both are good games but fall victim to their own size and the number of filler tasks that they both include. Origins is a tighter experience (better, music and story), while Odyssey has better gameplay and in my own personal preference a more interesting world.