It depends on the person. There’s a lot of consensus that the first AC, while good, lacks the polish of the later games. But it is amazing for it’s time. There is also general consensus that AC II is great, including Brotherhood and Revelations (PS3 and now also ported to PS4). Some people enjoy the first AC II and are less so fans of Revelations, but my personal take is that Brotherhood and Revelations are my favourite because The main character, Ezio, grows more appealing to me as he ages. Old man Ezio in Revelations is my favourite.
The rest of the game get mixed reviews:
AC III (PS3): a lot of people hate this game. Personally I think it’s one of the best. From it’s very thoughtful portrayal of indigenous culture to the fascinating ways you take part in the American Revolution, to the new found exploration of the wilderness, it ranks as one of my all time favourites. It can be buggy, but every AC has its bugs, especially from AC III on, but patches solve most of that.
AC III: Liberty (PS Vita & PS3): originally for Vita, and later ported to PS3, Liberty is the first with a woman as the protagonist. It’s a decent game that is bogged down by some interesting ideas that simply don’t work out that well. Its worth playing if you enjoy the series but it might leave you feeling less than satisfied with the overall mechanics and story.
AC: Black Flag (PS3 & PS4): sailing and pirating in this game is phenomenal. However the assassination component takes a major hit. The battle system was simplified to the point of being near broken. Counters and combos are easily interrupted by nearby NPCs, and even when combat works correctly it’s quite boring. I loved the sailing and exploration aspects of this game but don’t expect the most engaging story. It is by far the weakest AC in that respect.
AC: Rogue (PS3): a sequel of sorts to Black Flag for PS3. I haven’t played it yet but I hear mixed things. You play an Assassin turned Templar in this one which might be an interesting narration be switch.
AC: Unity (PS4): another I haven’t played yet. It was exceptionally buggy at launch, infuriating a lot of gamers. It’s since been patched and I know many people who consider it one of their favourites. It’s the first true next gen AC and takes advantage of the increased capabilities of the new systems. Things I’ve heard include: combat is much improved, navigation and cloning is much more fluid, retiring to the city is welcome with free climbing working much better in their game. It supposedly also has an excellent story.
AC Syndicate (PS4) another of my personal favourites. Syndicate fixes a lot of what was wrong with AC in previous instalments. Combat is much improved. Free running and climbing is a joy (thank you Ubisoft for the grappling hook). The story is excellent, especially additional DLC. And it’s the first with dual Assassin protagonists, and one is a woman which is most welcome. Evie and Jacob are excellent, with each having a specific focus or specialty (stealth and brawling respectively). Also London is fantastic to navigate and the Victorian setting allows for some superb side quests (solving murders anyone?).
I enjoy the AC series quite a lot, and even though I have a couple left to play, I feel comfortable recommending the rest of the games in the series to you. I’d say play them in order so that you get the full impact of the gradually building narrative.