I played up to AC3 and then felt like I had my fill with the series. I haven't played Black Flag, but I always hear that's the best one. I think the collection aspects in each game kinda just burnt me out. I spent so much time on the first couple of games that by the time AC3 rolled around, I was very much done.
Some key elements I really enjoyed were the environmental detail, the cat and mouse gameplay, which was also fun. One element I quickly grew tired of was tailing stealth missions, where you had to slowly follow the target to the end of the mission area. At the start of the series it wasn't super overdone, but boy did that get overdone in games since then haha.
I remember wanting them to go the whole ninjas route as it seemed like the obvious choice. So I'm kind of interested in trying the latest game at some point. I feel like we don't really get ninja games anymore, so I'm hoping it'll be good