As much flack as it gets for all being the same and coming out every year (not that these gripes aren't entirely without merit), I think it's a great, great series. I'd go so far as to say it's my favorite video game series of all time, knocking the previous king (GTA of course) off its throne.
I was really late to the party, as the first game I played was Assassin's Creed III when I got it in early '14. For whatever reason, the series never captured my interest before, but a Revolutionary War setting, a Native American protagonist, and a gorgeous countryside setting (someone was clearly studying their Red Dead Redemption)? I was in. And it wasn't the easiest start, to be honest - it took a while for me to get used to the somewhat odd control scheme and the whole thing just felt kinda wonky (oh man, you should have seen me trying to pick up and use a musket rifle in one of the early missions... my ineptitude put freaking DarkSydePhil to shame, haha). But the setting and story held my interest, and I loved playing as Haytham Kenway (he really reminds me of Timothy Dalton's James Bond, which is hardly a bad thing), so I stuck with it and slowly but surely got better. By the time I finished it, for all its issues, I found myself loving the game - and when I did a second playthrough the next year, after playing all the other games, I was worried that I wouldn't enjoy III so much anymore, but I found myself loving it even more. A lot of what some people dislike about the game - the really slow pace, Connor, the Colonial America setting - I really love, and while the gameplay stumbles from time to time (it's a bit too linear for its own good), I had so much fun running around the map and killing Redcoats with my trusty Tomahawk that I could overlook the flaws. And I absolutely love the story and character work, and was pleasantly surprised to find that it's a more even-handed, morally grey depiction of the Revolutionary War than the trailers that made it look like The Patriot: The Video Game let on.
So after loving III, I was definitely interested in playing the rest of the games. I asked for AC IV: Black Flag for my birthday and was just blown away. Not only was the story really good, but the gameplay was just a complete and utter blast - plundering ships and taking down forts never gets old, and being able to sail and customize my own ship had me nostalgia-ing hard about my Lego pirate ship from back in the day. And the basic assassin gameplay took III's formula and improved upon it considerably, and there's just so much fun pirate-y stuff you can do on the side. Those diving bell sections freaked me out though, as cool as they are. Da*n sharks and eels scared the hell out of me. I picked up the Freedom Cry DLC after beating the main game, and while it's a bit on the short side, the storyline and score were so POWERFUL that it more than made up for it.
Then I picked up the Ezio trilogy for cheap after hearing how series-defining and incredible those games were, and was not disappointed. For my money, II is the best of them - I loved how it took you on this big journey across Italy, as well as seeing Ezio grow from some street punk chasing girls to Master Assassin material. Brotherhood was also pretty awesome (and really made me hanker for an AC set in Ancient Rome), and while Revelations felt a little stale gameplay-wise right off of Brotherhood, I loved the story and setting. Between the two, I'd say Revelations is probably my favorite - while it's basically Brotherhood set in a different city, it feels more unique than Brotherhood, if that makes sense.
When the Unity/Rogue duo came along, I didn't have a next-gen console (and was in no hurry to play Unity, given its disastrous launch), so Rogue it was. While it's pretty much just a "Greatest Hits" collection of III and IV in terms of gameplay, I still found it incredibly fun, and I absolutely loved the storyline. As a huge fanboy of III and IV, this was like a dream come true, and Shay was a surprisingly great protagonist. And oh God, the Appalachian River Valley and North Atlantic were beyond gorgeous.
I cooled down on the series for a while, until I got an Xbox One bundled with Unity for Christmas. I haven't played through Unity yet, but I've played enough that I can say that I really like it. While the story is pretty weak and Arno's pretty meh, the gameplay is really fun, and Paris is freaking stunning.
Not long after, I finally picked up AC I after it became backwards compatible. And yeah, it's really repetitive, and feels more like a tech demo than a full-blown game. But the setting and story are just fantastic, and I really enjoyed finally playing as Altair in his prime and his journey from arrogant a**hole to more humbled and wise Master Assassin (even if the American accent was really unfitting). It's not my favorite of the series, but I'll never skip it in marathons, and I'm really interested in picking up that expanded Director's Cut for PC.
Then I picked up Syndicate, and that's the one I'm playing through right now - I'm on Sequence 8 (even though it's weirdly unlockable early on, I held off on tackling it until after beating all the other Sequences), so I'm not far off from beating it, and I can safely say it's one of my favorites in the series. The story may not be quite as epic as say, II or III, but it's still really fun and has some great characters, especially Evie. And the gameplay is just so much fun, with a wealth of stuff to do, and really enjoyable stealth (one thing the series has always struggled with - I'm still kind of amazed that it took them until III to realize "Hey, maybe we should let players lean against a wall"). And London is just amazing, probably my favorite video game city since GTA IV's Liberty City. I can't wait to tackle the Jack the Ripper DLC, I've always been fascinated (and creeped out) by everything surrounding him.
As for where the series goes next? If the rumors are true and we get an Ancient Egypt game with Empire, I am completely on board. Ancient Egypt is one of my favorite historical settings ever, and given how far back it goes in history, it would make for a really interesting story. And if it's expanded into a trilogy with games set in Ancient Rome and Ancient Greece? He** yeah, I am so there.
EDIT - Thought I'd edit my post to give my thoughts on Liberation, since I realized I forgot to bring it up (I played the HD version off Xbox Live). It's pretty fun, I loved the setting, and the story was pretty cool. Aveline was an awesome protagonist. And being an AC III/Connor fanboy I loved the tie-in mission, haha.