AC 1 was repetitive but the parkour gameplay was unique at the time, a new way to control a character, in a more "realistic" way compared to prince of persia sands of times trilogy - some people used to claim assassin's creed was a "spiritual sucessor" of prince of persia. Also, it was one of the first big releases for "next gen consoles" PS3 and x360. The graphics were good, and I believe the marketing was heavy, and the game was presented as having multiple ways of completing missions in a deus ex kinda way, though it was a lie.
AC2 improved a lot because it is probably the least ubsofty open world game made by ubisoft: It follow a structure similar to the good GTAs, where you had varied type of missions instead of same missions ever. Every mission point would present you with a cutscene in the same vein as GTA, and missions would appear on the map by progressing through the game, while in normal ubisoft games they appear by unlocking hub towers. Brotherhood was also good, but revelations was a cash grab while they were developing AC3. Also, the subject 16 puzzles through ac2 and brotherhood were cool.
AC3 disappointed me in three ways: First, Connor was a terrible protagonist, and to make things worse, you play his father which is a far better main character. Second, The grinding, which was the worst: hunting animals, waiting for NPCs do something different for you to witness in your homestead, it takes forever to gain money to upgrade your ship. Third, the "modern ending" which was silly as hell. Until revelations, the so called "those that came before" were ghosts of the past and the main threat was the templars. when the end indicates that the great threat was juno being ressurrected, that was the silliest thing ever. And they had a good plot in their hands since AC1, which the templars planned to use the "apples" to control humanity by putting them in orbit of the earth, and that would be a great way to save earth from sun explosions, but with the caveat that the templars would win. that would be a better excuse for sequels also, but they prefered to bring back aliens and make us believe that would be worse than the sun ravaging the surface of the earth. also, haythan kenway even mentions in game that trying to discover these ancient secrets were silly and the templars should follow a more down to earth plan and find other ways to keep power and maintain control over humanity.
But, aside from this, AC3 is a good AC game, and introduced the sailing ship gameplay that was fun as hell, Haytham Kenway was great, and though the setting was boring, it was as varied in missions and things to do as ezio's games. Though sometimes it reminds me of AC 1, with a big wilderness area, but at least it was not empty as the one in AC1.
AC4 is more like a spinoff, which took the ship gameplay of three and made a game out of that, with a city or two just for some parkour. Rogue I didn't play.
AC5, or Unity, is the worst of the games. Too ambitious and offers nothing. I didn't even had the glitches/bugs people experienced, but it was cumbersome. every mission was the same: Kill the templars, only now you had tpo go to the other side of paris to kill them. the story had so many look alikes that it was hard to tell what was going on. this game introduces a shit vision that was revealed to the player when he killed the main villains, as if he could see what his victims did. It was not explained whatsoever how the main character could achieve this. Oh, french revolution? doesn't play any part in the plot. The city of paris is very big, and probably the most detailed AC city, but its horrible when you see years pass, and the same crowd protesting in front of some palace, or guillotine executions in some plaza that happens everyday for years... Oh, what about the crime mysteries that had lots of clues that could mean anyone could be the criminal and in the end you would eventually accuse the wrong person? The DLC at least was better maybe because of underground areas that reminded me of catacombs and chrch interiors from AC 2 and brotherhood.