Just one? The one that offers the most versatility in terms of the features you want.

The one that offers the most potential fun is my suggestion.

depends on what you consider 'fun'

are people you want to talk to 'features'?

You know, seriously, I don't think there is one ideal social network out there yet. I think there will be a better choice down the road.

The idea of different circles of friends, family etc of google+ is a great idea, let's see how they implement it.