Brotherhood was a fantastic game, every bit as good as ACII and the assassin recruitment angle was a welcome addition. Revelations definitely felt like the formula was running out of steam, but the gameplay was still pretty solid and it gives Altair and Ezio a touching send off at the end. I adored Ezio as a character, and I'll miss him.
I really don't think it's fair to dismiss AC:B and AC:R. They may not be numbered sequels, but they're both worthy entries in the series canon.