With Syndicate only out for a few days now it's tough to make a real call yet as to where it really lands in the ranking of the series, as viewpoints are going to change as a game sinks in over repeated plays. Assassin's Creed III is probably the best example of that - it received stellar reviews from the major game sites at launch but is now universally reviled.
We'll have to see with AC:S finally lands, but right now it seems to be sitting in the middle of the pack: there's some great stuff going on, but it certainly isn't the peak of the franchise. Carriage chases and top hats aren't quite as innovative as being a pirate.
As a standalone, side-story title it gives a brief glimpse into Victorian era assassins, again slightly changing the formula and abandoning multiplayer, but leading some to wish for another multi-game arc featuring a character we can love as much as Ezio.
Personally, I'm a fan of the top hats, mutton chops, flintlock pistols, and high speed carriage chases. With the organized crime aspects and shooting from a carriage while chasing down other horse-drawn vehicles, sometimes the game almost feels a bit like you're Nico Bellic in old time England. It probably won't ever be heralded as the best entry in the series, but it does sit solidly in the middle.