By the time Assassin's Creed 2 came out, I was calling this my favorite series ever. The Ezio games made you feel like an expertly skilled badass with unprecedented success, and the stealthy, agile archetype is my favorite to inhabit. Even the gameplay loop was fresh and borderline revolutionary, and so successful it became the basis for what would soon be tarnished as the Ubisoft blueprint.
Last game I played was Odyssey, and though I spent...a lot of time on it, I've never felt more bitter at the end of the game. Seems you only get half the conclusion by completing the story missions, with the other half locked away behind the assassination list. And since any enemies 3 or more levels higher than you are always essentially indestructible, and the assassination targets climb very high in levels, it's essentially a driver for bottomless grinding. By that point I'd already had far more than my fill.
I was casually interested in Mirage, but I learned from Skillup's video that it's essentially an upscaled Valhalla DLC, which is a red flag. With other reviewers pointing out that it, well, feels like upscaled DLC, I've no interest personally.
I loved 2 and Brotherhood, but 1 and Unity were the closest this series ever got to delivering on the elevator pitch, especially Unity. I tried Odyssey and had much the same experience as you, except I didn't have the patience to stick with it once I could see where the grind was headed.
You know one of the reasons I jump to 2, and I didn't think of it until now, is because of aerial assassinations. If I remember right, there wasn't an actual mechanic for it in 1, and the addition in 2 and beyond tied things up nicely.
Yeah, and the investigation missions in AC1 weren't great, but the assassination missions gave you the freedom to scope out a mission and do it yourself (same goes for Unity, even acknowledging in the dialogue to the player character that it's something the series has been missing), which was a thing that AC2 took away in favor of set piece boss battle moments.
If anyone enjoyed the earlier AC games, but avoided Unity because of bugs or bad press from its admittedly disastrous launch, all of that is mostly ironed out (at least on PC), so give it a shot.
It holds up surprisingly well, honestly, and probably has the best movement and parkour in the entire series.
Plus Revolutionary Paris is one of their best environments. Like, I like them all, but Paris is particularly well-done (I'd make a joke about Notre-Dame being well-done as well, but that was more of a medium-well result).