I still think AC1 is still the worst one of the club. While AC1 was a new IP, a lot of the series' problems of the AC series began here. The biggest problem is probably the cut+paste issue of voice-acting + side-missions types - AC1 absolutely takes the cake for this one. AC1 set the trend for (often) bad + anti-climatic endings that many of the games unfortunately followed with, in most instances - i.e. AC1, AC2, ACB, etc. Let's also not forget how the game, for the most part, is a stealth game for the 1st 1/3rd, but then suddenly in the last 1/3rd becomes a back-loaded action-fest, which this game never ever needed. Plus, this game on the PC ran like garbage on most PC's at that time when it was new - yes, including mine. It was a cool title b/c it was a new IP for the time, but extremely flawed.
I think AC: Revelations was the start of the fall of AC series - and is often where I place the game that belonds next-to-worst AC game. TBH, the series really didn't bounce back until AC4: Black Flag when more different types of traversals (namely Pirate ships!!!!) besides running, jumping, and platforming everywhere were introduced into the game's core+main gameplay (among other things). Of course, AC Unity wasn't anything special either b/c it tried too much to be like earlier AC games in design+gameplay, but it certainly bounced back again with AC: Syndicate b/c they learned from Black Flag that different traversals can break-up the typical AC problem (i.e. buggies, carriages, etc).
AC: Revelations might have one of the better stories, character developments (i.e. this was the game where actually Altair's character became less of a jerk + had more dimensions to it) & it wrap-ups b/c it ended both Ezio + Altair's story, pretty much - but the new game-play type of the side missions absolutely sucked here - the Tower Defense mission + the Portal-like puzzles were poorly executed & put together.
AC3 certainly wasn't anything special and suffers for a lot of "too much of the same" in the gameplay department, but its story is absolutely over-the-top + ridiculous (Connor showing up at just about every important meeting for the Foundation of USA + every major important event in early Boston history); and Connor is easily the worst character in AC history. Connor could've been interesting, given his background + the opening events of the game - but just like Aiden in Watch Dogs, UbiSoft did nothing w/ making Connor take a stand for much of anything & is just a flat-out dull one-dimensional character. At least the setting of Boston, NY, and whatnot during the foundation of USA was a really cool + interesting setting.
TL;DR:
I have it look like this -- AC2 > ACB > AC4 > ACS > AC3 > AC Rev > AC1.
ACU, unsure where it belongs yet; never finished it.