1. Assassin's Creed: Syndicate
Much more refined than previous games, great assassination missions, beautiful world and setting, good characters.
2. Assassin's Creed II
Absolutely loved it, great setting, amazing cities. Also loved the absolutely preposterous present day stuff in here.
3. Assassin's Creed: Brotherhood
More refined than AC2, the extra assassins are great fun to use (if a bit strong), but renaissance Rome is not Venice or Firenze, which is why it ranks below AC2 for me.
4. Assassin's Creed III
Only just edging out Unity for me and only just behind Brotherhood, this is a game that every time I replay it I enjoy more. Where Revelations (see bottom of the list) is a game that absolutely wastes what should have been an amazing setting, AC3 does pretty much the opposite. Building the Davenport homestead up was great too. Ironically, the only major problem I have with the game is the thing most people like about it, the naval stuff. I think it's poorly executed and simply not very fun.
5. Assassin's Creed Unity
Far and away the most beautifully realized city in the franchise, and some fantastic assassination missions. However not nearly as refined as Syndicate and Arno is a relatively bland main character. I should note that in contrast to most, I didn't experience any bugs in this game and it ran fine, but I recognize I was lucky and many other people have a very good reason for ranking it lower.
6. Assassin's Creed Rogue
I kind of wish I'd played this before Unity rather than after, both because this game sets up Unity and because this being a last gen game, the visual difference was rather large. That said, this is a nice send-off to the AC games set in the Americas. It was also very nice playing as a Templar for most of the game for once. I thought the world was at times a bit repetitive though, and the naval game while less of a focus here, is still not that great.
7. Assassin's Creed Liberation (HD)
Honestly, most of the things that make me rank this game this low are things down to the original platform it appeared on. There's only so much you can do when porting from a handheld. Missions are much shorter (and often chopped up into bits to make them so) than otherwise, visually even after upgrades it's not great, environments are relatively small. That said, I did like that the game took more risks than most of the mainline ones do.
8. Assassin's Creed IV: Black Flag
The opposite of AC3, I enjoyed this game much less on replay. This also ranks quite far below the others mentioned so far. Mission design is some of the weakest in the franchise, I don't care for the main character or most of the side characters, naval stuff gets tedious quick, your home base is quite uninteresting, and I don't care for the world all that much.
9. Assassin's Creed
Another large gap between this and AC4. Great in concept, fantastic setting, but the buildup to the (rather good) assassination missions is very repetitive and in general the game is pretty clunky all-round.
10. Assassin's Creed: Revelations
Ranking far, far below any of the others for me, this game really wasted what should have been one of the most amazing cities in the series and turned it into a bland, dull and tiny hovel. I kind of enjoyed it when first playing it, but on replay I got annoyed by just about everything in this. They really shouldn't have stretched Ezio's story this far. I'd rather they'd have taken a year off instead. Hopefully, at one point Ubi will decide to do Constantinople/Istanbul justice with a new game, but I feel this game might have killed any chance at that. Oh, and of course there's that terrible tower defence game.
