I played AC2, thought it was a great game... tried to play a number of other and just couldn't do it... they all felt too similar to AC2 without the initial novelty.
ACII was initially my favorite because of how amazing the setting was in Renaissance Italy, but ACIV really changed the game, which is why I liked it, and why it might be my favorite overall.
It still had the formula of "collect these things for the best armor" and "get to these lookout points to view the map," but those things were definitely reduced majorly in terms of grinding/importance.
It wasn't just about running across rooftops, climbing a building, killing someone, rinse, repeat. You sailed, had naval battles, controlled a fleet, went to big cities, tiny ports, even smaller forts and villages on small islands, dove underwater at ship wrecks, and on and on.
You could go into a big ass fort guns blazing, or sneak through the Mayan ruins like a stealth master.
ACIV reallllllly developed a game that let you play how you want, and in ways much different than the rest of the series.