Love AC. I liked all the games I've played so far and I'm waiting for AC6 with a burning passion.
From what I've played
AC3 -- Probably the best designed of the bunch. Every mechanic just fits so damn well, and the story is not totally shit for once.
Silent Line -- Definitely some mechanical improvements on AC3. I like it, but it's way smaller than the previous game and the story is way weaker.
Nine Breaker -- It was ok.
Last Raven -- Terrific core gameplay, horrendous execution. Very rough diamond.
AC4 -- Solid entry, but extremely rough around the edges
ACFA -- Way better than AC4, but I feel like there were some things AC4 did better. I really hated the 3 paths system. This is my favorite AC game, even if I think that AC3 is miles better from a game designer's standpoint.
AC5 -- It was interestingly fresh, but like AC4, there was some clear room for improvement. Solid stuff, though, and I think it's one of the best AC games in terms of level design aesthetics (as far as technical graphics go).
VD -- Good entry, but I'm kinda bummed that From Software failed to capitalize on UWs, and the level design in general. The levels never really felt like they fully utilized 5th Gen's core gameplay style. Also, making the Unidentified Weapons multiplayer-only is a real waste. Loving the UNACs, though, even if they're completely useless on some maps and against any melee player.