Hmm, really hard question then. If we would just stand somewhere in one of the cities (in the game), and simply look at them, then I would say that Driver: San Francisco seems to be the most lively. The first reason for that would be that there's much more traffic than in all Driver games, and also much more pedestrians. Also when you look at the pedestrians, they look much more like they have a life, compared to those in other Driver games. They carry bags and eventually let them fall, and they all look much different (in Driver 1 there was like what, 2 pedestrians?
).
You can also see ships passing by and other objects that move, such as lifts on the outside of buildings for example.
On the other hand, there's stuff in Driver 3 and PL that isn't seen in DSF. For example cops that come when an accident or shootout has occurred.
In Driver 1 you had planes flying over the city, which I found pretty cool.
Then obviously, in all other Driver games you have real traffic lights and such, but I'm not sure if it contributes something to how lively a city appears, I guess this has more to do with realism.
The only real things in the Driver series that make the cities appear more lively are: vehicles, pedestrians, objects. I say objects because in DSF the streets are full of stuff, whereas in other Driver games the streets are a like a desert, the most you will find are probably a light pole and a letter box