I only played the last three of the series, and the one I prefered among those was Revelations. The city was perfect for Ezio to jump everywhere, and you had really useful weapons like bombs or Axes that you could throw (plus the fact you were able to kill passants ).
III had a really good story and a really long life, but that was all, the city had too large street to enjoy jumping on buildings.
IV is a really good game... for a Pirates of Caribbean game. It has interesting cities but you spend 70% of your time on your ship. And it has a very short life.