Opinion is overall divided here, with fans about split as to which game is really the best, and I have to admit even I waffle sometimes on which is currently my favorite. Honestly, this and the next slide could be swapped on any given day and they'd probably still be right, as both Assassins Creed II and Black Flag are very solid high points. But, looking back across the entire series, the sequel to the original title just does so much right that it's worth being counted as the pinnacle.
While the original title offered a previously unknown mix of historical stealth combat and modern day sci-fi shenanigans, the first direct sequel improved on nearly every aspect in major ways. There was no more running back and forth from the safe house constantly, significantly improved combat, better storylines, and the most loved protagonist in Assassin's Creed history: Ezio.
The dynamic environments for fleeing (or stalking an assassination target) still hold up today even after so many iterations, and there was tons of fun to be had hunting down all the feathers, video segments, and statuettes. Granted, by today's standards it may not have the graphical flair (or the ship-to-ship combat that's become so highly acclaimed), but this is still one of the most fun, polished games in the entire series.