1 is an interesting concept but the missions get repetitive fast. II and Brotherhood take the concept from 1 and polish it into excellent games.
Revelations is good (I am a minority in this opinion) but it really adds nothing new and takes out a few cool mechanics too.
III is arguable as bad as I.
I highly recommend II and Brotherhood.
Could you elaborate on this? Was planning on picking up AC3. Loved the hell out of Brotherhood and II.