Rogue reused assets, but didn't lack soul, not since it was a rushed shoehorn title, which was still pretty good.
Ship combat was improved? I mean it did have some different mechanics, but it was unbalanced as hell.
AC1 is dull and borring besides the intro and the ending. The story is okay, has some good dialogue, especially with Al Mualim, but the gameplay is so repetitive it's a chore to play it midway.
Brotherhood was fine, Black Flag was great.
Origins is overrated. It's literally a shorter Odyssey with an even dumber skill system and in between the two or three really, really good story parts, especially the one with Caesar, it's a fucking mmo slog, like you say.
I'd say to get a good AC expereince is to play the Ezio Trilogy, including AC1 if you can spare 10 hours for it. And the Kenway saga, maybe even play it in historic sequence starting with BF, then Rogue, then III.
I hear Unity has good gameplay, never played it though.
The rest are MMORPG clusterfests.