Please
I figure most people have the 2nd one and are missing the other considering they were the first to go entirely
Personally I liked all of them before they changed the style in AC Origins (the Egypt one), and even that was playable, Odyssey was an improvement on that even...but neither is really the same game in general as the ones before Origins.
The only two I didn't enjoy much were Black Flag (matter of preference) and Valhalla.
Black flag you sail on a ship repeating raids on islands, searching the ocean for stupid items for completion if you care and getting caught in ship battles...which are fun for the first hour or two until you get some good upgrades on your ship...I honestly don't see why the game is so popular unless people like playing battle ship all day mindlessly...which i have to figure is the case because they cried for a ship based game for so long even Ubisoft listened (game REALLY sucks btw, I got to play the early beta)
Black flag you might want to play (or watch the scenes at least) for story sake, but I had my save erase at 99% completion after scouring the ocean for stupid fragments because the game is all ocean filler just to miss a platinum trophy because of not being able to take down a f'kin single super boss ship out of the 4 at end game before it happened. You basically spend all your time either on a boat or running through asset copied little islands 75% of the game, although the story itself is decent.
I suggest you at least watch some of that game in the middle before you waste your money and decide if its for you.
I would avoid Valhalla like a plague, it still has bugs I doubt they ever plan on fixing...assassination, which they supposedly brought back is a failure, you can't hide when your hidden...you might as well play God Of War with an underdeveloped character while you (once again) sail the world on a ship...but this time you have a tiny boat with oars
If you enjoyed the first one at all, excluding the repetitive factor and prefer stealth and actual assassination style gameplay instead of wishing you could actually fight with your sword all the time and stop hiding, then I suggest you play them starting right from part 2 forward.
None are even half as terrible as the first in the fighting sense, you can actually survive combat much easier in any of them after that, as well as walk freely for the most part (if you choose), stealth is more optional with each newer game they released.
Until Origins you can sneak up and assassinate
any enemy you come across and play the entire game with very minimal combat (usually just to teach you how) and walking straight into enemy territory is always a bad plan...again, becoming easier and less suicidal the newer the games get, but never outright necessary except a few missions.
If your not sure which you prefer, stealth or combat, I would say play Assassins Creed 3, it was the first with all aspects of combat that the series uses until Origin but you can also play almost pure Assassin as well, its the best of both worlds.
If you like the combat factor, play the ones after it...if you enjoy killing quietly, definitely go back and play the Ezio collection which starts with part 2, thats the way Assassins Creed was meant to be IMO and the best of the entire series as well.
Brotherhood and Syndicate are good combat versions, but still true to the design and original style and both are extremely good games, but once you play those the older Ezio ones will feel sluggish and remind you of the first game again because they have much more simplicity in combat and safety in motion (no more falling to your death lol)
Its really hard to say because I don't know exactly what you enjoyed in the first game.
I would say either try part 3 to figure that out or play part 2 and if it irritates you like the first, go to the next one over and over until you hit a nice balance...but if you don't like Part 2, or even Brotherhood, your probably going to prefer the newer titles like Origin, because the way this game was meant to be played really isn't for you.
The one that was just released actually looks more like the classics, but I haven't seen enough or played it myself to be able to recommend it even if it is...and if you want to enjoy them all, any modern one will make the old ones feel hard and slow in comparison.
Sorry for the long rant...I literally bought my first PS3 for this series, when they changed it into a mockery of what it was i actually would have preferred they released a new series entirely and let it die with some dignity...hopefully they actually go back to the classic gameplay and salvage whats left, but time will tell.
You game is on the way.