Which AC game should I play next?

As per Sanunes I second Black Flag, Edward is a very different protagonist and a bit of a rogue and not really a typical assassin. The story is good and the NPC's are good and the setting for the time it was made is environmentally quite pretty and scenic.

There is a fair bit of sailing though which takes a short while to get to and a fair bit of resource purchasing/gathering to upgrade your ship and gear.

Unity is a pretty game with an OK story but the outfits, weapons, parkour and cmbat are nice and fun. Paris is gorgeous and fun to explore and the historical characters are good. Arnuo (SP?) is my least favourite AC protag.

Syndicate is one I didn't really like but a lot of newcomers to the game seem to really like it, London is well done but the base NPC's that inhabit are just cloned. You switch between twins as protagonist Evie and Jacob and they have slightly different skills and abilities. I loved Evie Jacob not so much.

AC3 is a bit unloved as Connor/Ratonhnhaké:ton is not the most engaging/relatable of a protagonist but for me he grew on me and matured a lot as the game progressed. I also loved the historical setting and the story and especially exploring the wilderness areas and I liked developing the Homestead and hunting. Some QTE's to negotiate.
 

$10 dollars is not much of a loss these days if you don't like it, I hope you do the combat is fun and the gear I found really aesthetically nice.  The parkour is probably the best of any of the games I hope you really enjoy it.

The older AC games all seem to go on sale a lot. I got all of mine steeply discounted except for the ones I bought new, so you can acquire the rest for really good prices if you look out for those sales.

I got about halfway through Unity before taking a break (not Unity's fault, just wanted to play other games), and I found it really fun. I did the co-op mode with my brother several times and it was extremely buggy, but I didn't have major problems with single-player mode (nothing game-breaking, anyway).

Syndicate was my first AC game and I loved it. The mechanics are very similar to Unity. AC3 is very good, too. The trading mechanic was confusing and I didn't use it beyond the tutorial missions for it, but I thought both protagonists were very interesting.

If you end up liking the older style of AC game, it's worth going back for the Ezio trilogy too.

I ended up spending a lot of time hunting and was able to make enough money for what I needed It's probably quicker to make trading work, but I couldn't be bothered.