What would you say is the most complete game in terms of gameplay and mechanics?

For me no game in the series is a perfect 10, all of them have pros/cons and no game does everything 100% correctly. Every game does something that others don't and vice versa, they never managed to keep the best aspects of every game and put it into one definitive game. In most cases they went one step forward and two steps back.

I think that Brotherhood is one of the most complete games in the franchise and definetly the most fun to play among the first 4 games. It perfected the original formula and every system and mechanic came together perfectly. Parkour, combat, tools, chain kills, stealth, Assassin recruits, outfits, economy system, side missions. I always have a blast playing it.