I say "again" because I got Black Flag when I purchased a PS3 years ago but I never got to really enjoy it. Maybe it's the game? Maybe it's the controller?
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Black Flag was my first Assasisn's Creed game and it was wonderful. Fun setting, fun combat, simply but interesting story that really felt like an adventure. But the problem with the game was the controls were utter trash. You can be running straight and have your shoulder rub against a wall and next thing you know your character is halfway up a building and there is a delay for stopping. Then you get hacked by a bunch of random people who can also climb and parkour just as well as you. The controls were horrendous. Which is a shame because the game was brilliant otherwise.
The games, when the controls aren't trash, are fun. Rogue was like Black Flag, just not as unique or fun but the controls were fixed. Unity was a mix but still fun. Syndicate was better control wise, but got repetitive and the central story was sorely lacking. 3 was utter horse crap and never should have been released.
Apparently after finally getting things right with Rogue and Syndicate, they redid the controls and made the game more grindy and drawn out from what I can gather. There are level recommendations for certain missions apparently and if you are too low it will be very difficult... so it sounds like you will be forced to replay not so well done repetitive side quests to progress. So personally I have kept my distance because I don't have faith in them doing an excellent job.
If you still have Black Flag give it another go and then consider trying this one, IMO.