So you've played Unity then. I heard it was.. less than impressive.
Cool, hadn't met someone who thinks Black Flag is the best one yet. I heard you either love the series from AC1-AC3 or AC3-future. Not both.
Well I didn't get it at launch or on PC, so I never faced any technical issues in the singleplayer experience, as the problems were all patched not long after it launched. I don't know if the PC version is alright or not, but I'm sure that doesn't matter to you. All that matters is that it works fine, and it's all fixed up. There are a few little tiny online bugs, but it could be much much much worse… LITTLEBIGPLANET 3, I am looking at you!
My guide to the series would go as follows:
AC1 is pretty short and simple. The characters and colours are pretty typical and boring, although I personally like the era so I was a bit more motivated to play through it than most people really should be in our age of modern gaming.
AC2 is a proper sequel. It has still got a very limited colourscheme, but the characters and story more than makes up for it. Not only this but the combat is pretty good now. Sure the economy is pretty easy to break, but overall it's a pretty engaging game that doesn't really require you to have played the first one.
AC: Brotherhood is just more of Assassin's Creed 2, so while it may start to get tired near the end, it is all pretty alright.
AC: Revelations is a good way to wrap up everything that happened with our main man from AC2 and Brotherhood. It has a bunch of callbacks to a vastly improved version of the first game's protagonist, which was pretty well done to me. It's not the sort of game to dro and b=pick back up later, because then you can get pretty confused with the three different plots you're following. Oh and the gameplay is more of the same.
AC3 is a new ear and a new cast. This time in a more seamless and vast world. It even has seasons, so if you don't want to look for the collectibles in Winter, just play the story up until it's Summertime. The game is fairly long, and with a fair share of twists and turns. Its a bit of a shame that the setting and background story is so confusing and messed up. But that's the problem with trying to be historically accurate. Oh and this is the last game to ever care about "the present". Most people liked that it ended here, but I personally miss playing in the present.
AC4 is a brilliant pirating spin-off. You play as a pirate, and the story follows the typical GTA arc, with the protagonist unsure of what they want to do with their life, so they decide to join the pirates and work their way up to king of the pirates, because the British and the Spanish are all greedy buggers than need to be brought down a few pegs.
AC Rogue is AC4 but with a blue HUD and set in AC3 and the Atlantic Ocean (I think that was the name of the ocean, but I'm not a massive geography whiz).
AC Unity is the next step up from AC3. It's pretty good, and I think the combat is more skill based than ever before. The climbing is improved too. The only downside I see is that unlocking items tends to be a bigger grind now than ever before.
Graphically AC4 and Unity both look stunning.