Wait...wait, wait, the other in-between versions of AC I understand how anyone didn't like them but Assassin's Creed IV: Black Flag & Assassin's Creed Syndicate were DISAPPOINTING TO YOU?!?? Huh?? But then you ended your statement about having fun "since Brotherhood or 4"??
Please clarifiy, my good man, I am interested to hear.
I'll just do a run down my opinions of all the main games:
AC1- Repetitive but interesting, felt like an actual Assassin.
AC2- Best game in the series, Ezio is a wonderful protag' and renaissance Italy is such a beautiful setting, still one of the best in the series.
ACB- Good follow up to AC2, maybe the second best game in the series.
ACRev- Meh, very forgettable.
AC3- Great story, boring game play and setting.
AC4- Super fun game-play however the game just didn't feel like Assassin's Creed to me. It focused too much on the pirate shit.
ACUnity- A decent enough game but it was buggy and had micro transactions . It's really pretty though.
ACSyndicate- Very uninspired story, so much potential for a good story during Victorian London, a industrial revolution, and they went the worst possible route in my opinion. A one-dimensional villain and an idiot protagonist the game forces down your throat. It played well but that was pretty much it.