I've played almost all the mainline AC games, so not counting the Chronicles games; Also a few hours of Liberation. I played them all up to Unity as they came out. Unity is when I finally started getting burned out. Looking at the map in the beginning, I just didn't have it in me. And it took a couple years before I came back to it and finally finished. Now, ultimately enjoyed Unity.
When I finally got to Syndicate, I got so frustrated with the combat that I almost gave up. I wanted to like the game and I am stubborn, so I watched some videos for combat tips, and I finally got the hang of it. I also wound up enjoying Syndicate.
So, I'd say even though I enjoyed Unity and Syndicate, they were the two that almost broke me from playing one of my favorite series. I did start getting a bit tired of clearing the areas in Syndicate and didn't care of how you unlocked some of the gear, or upgrading your equipment. Also wasn't crazy with some of the side missions. So, I wouldn't say it is the worst, but Syndicate is probably my least favorite of the AC games I've played.
With AC3, I liked it quite a bit, when it came out. I actually liked fixing up the village and talking to the people. Granted, I haven't played it since I beat it not long after release.