Hard to order them so unequivocally...Don't feel like any one game does everything better than the rest. there were certain aspects of AC1 that are still unmatched in the series, but they obviously didn't have the formula completely figured out back then.
AC3 is the newest and should be the most polished, but in a sense, it's almost as unrefined as AC1. It does have far and away the most content though, and while the gameplay changes are a bit iffy, it's still generally an improvement.
The Ezio chapters are easy enough to rate: AC2>ACB>>>ACR