The last AssCreed game I played was Brotherhood and mostly played them for the story. I have a PS4 (and PC) but is it worth playing the new ones (Black Flag + Unity) if I've missed a chunk of them? Are the stories decent at all as that's what I usually came for with the series.

AC2, Brotherhood, and Black Flag are the only ones worth playing. That 2D one from Climax seems alright though. There's a bunch of quick looks here that could help you make a decision.

Black Flag is great. The other games you've missed since Brotherhood are entirely optional.

Yes. The games after 3 cop out of the story and are pretty much unrelated to the events of Desmond. So, you can play Black Flag without ever worrying about loosing some important part of the lore...

In terms of games, I think Black Flag is the only ones of the new ones that is worth playing. Haven't tried China yet, so it might be interesting, but the other "true" AC games are not good.

Black Flag, no questions.

Okay, so I haven't played all the games, but I live with someone who is an avid AC fan. He has played every entry, all the DLC, read the comics and collected a good amount of merch. So, by osmosis, I have seen a TON of AC games.

I really enjoyed all of the Ezio games (Assassin's Creed 2, Brotherhood and Revelations) and Assassin's Creed 3 was pretty cool, but I can totally understand skipping it. The protagonist is a totally different personality type, and can put people off. If you are interested in Desmond's story, play it. If that doesn't matter so much to you, I would say it's not a necessary entry. Assassin's Creed: Black Flag was very cool for the boat fighting and whatnot, but the story wasn't so interesting and the main character was like a discount Ezio to me.

Out of the newest games, Rogue and 4, play Rogue. It had a cool protagonist, interesting story, and was a better, more playable AC game than it's newest gen counterpart. I really liked the story beats in Rogue, and there were a lot of cool callbacks to 3 and Black Flag.

The thing you have to realize is that 3, Black Flag, Rogue and 4 all have storyline connections, so skipping one may lead to a story beat in another game falling a little flat. In fact, a big dramatic moment in Rogue may completely do nothing for you if you skip 3. If you only choose one or two to play, pick up Black Flag and Rogue, both were good AC entries.

Black Flag is worth playing even if not for the AC stuff...Black Flag is just a damn fun game.

Black Flag is really fun until you set foot on land. People seem to forget or choose to forget how it has the absolute worst on-foot missions in the series. A whole lot of tailing missions and uninteresting architecture.

Don't get me wrong, all the boat stuff is very fun. Upgrading it, hunting whales, fighting bigger and bigger ships - that part of the game is awesome. All the storyline missions are pretty bad, like worse than your typical AC type missions. AC3 was a mess in many ways but I feel like at least they tried to do something different every once in a while.

EDIT: People like to hate on it a lot but to this day the best playing AC games was Revelations. It has the most refined and enjoyable gameplay in the entire series. The hook you get might seem like such a small thing but it really added a lot to the traversal and I really missed it in future games. It features some great architecture and some really great climbs up remarkably tall structures. It also featured some really fun Prince of Persia inspired tombs to explore and probably had the best feeling multiplayer.

It's a shame no one ever mentions it.

Black Flag is really good, AC3 has the best multiplayer but I don't know if anyone's playing it anymore.

Unity is... Well, it has a bunch of new work done on some of the basic movement and assassination missions that are interesting if you want a look at what future AC games will probably play like, but I would agree with the rest that if you only play one AC game after Brotherhood make it Black Flag.