Identity EASILY for me.
Identity had the heart pumping action, chase scenes and fight action. The wide eyed discovery that he's some kind of assassin coupled with the slowly unraveling conspiracy make this one the most tense by far. The romance was an excellent slow burn, and Bourne's acceptance and reluctant opening is well played out.
In the last two, he simply becomes this emotionally barren superhuman man on a revenge trail. The action wasn't quite as fun as Identity, despite being more visceral.
The second just felt like a poor filling in a sandwich, whilst the last was just empty for me.
The first had some great locations, but the second and third seemed to simply regurgitate glass building after glass building. The only parts that really stuck in my mind during the third one were the scenes directly ripped from the first (the hair dyeing scene and the body floating in the water).
BI for me all the way.