The first game has a weird gameplay loop where you get to a city that is very similar to the previous one, have to do a some filler missions (often with no story at all) to unlock the story mission, then do the story mission and move on.
2-Syndicate are much more continuously story-driven. They all have quite a few collectables, but they aren't important to experiencing the game.
The 2 family is mostly set inside cities, while 3 and after have more world around the cities. They also lose some focus on stealth over time, though it still exists in all of them.
Origins, Odyssey, and Valhalla become much more RPG-lite, combat focused, and require you to do quite a bit to keep up with enemy level scaling.
Looping back to the root of your question, the 2 family is often seen as the peak of the core series, with 4 (Black Flag) being up with it but different.
The only downside of the 2 family is that there isn't much evolution between the three games to make moving to the next game feel like a jump to a new game, but progression is lost each time. It feels like one massive game with weird break points.