Each game in the series has its own unique strengths.
AC1, it's the openness to experimentation and the sci-fi frame story.
AC2, it's the Ezio storyline and historical backdrop.
ACB, it's the focus on exploration and side missions.
ACR, it's the refined mechanics and emergent city environment.
Also for AC2, it's the puzzles-within-puzzles-within-puzzles -- the super-hidden material in the glyph puzzles -- that actually make you, the player, go out into the real world and do some research to solve.
You use a controller to manipulate Desmond, who uses the Animus to manipulate Ezio, who finds the glyph puzzles for Desmond (controlled by you) to solve, and to find the deepest stuff in those puzzles, you have to step all the way out of both the Ezio layer and the Desmond layer and start Googling to learn the Masonic code, or the Caesar cipher, or any of the other crazy stuff that helps
you figure out what's really going on in the world of AC2.
This interactivity between game and player on multiple levels is what made AC2 an amazing experience.