It obviously used to be Crash Bandicoot but that was a decade ago and those days are over. Then it seemed to be Snake despite being third party but he's been in at least one MGS for the Gamecube as well as being in SSBB and maybe eventually another game on a different console so it can't be him.
Kratos would be an obvious choice since his character is widely known in the gaming community, games sell very well and also exclusive. The violence though makes it hard to make him the face of your company if you're trying to reach out to a wide demographic. Gran Turismo has been their most popular franchise over the years by far but it's still limited in demographic since not everybody likes racing and a car doesn't seem that iconic unless we're talking about KIT(?).
Sackboy is obviously a very new character and who knows where he goes from here. So that makes it hard to consider him the face so soon but maybe in the future after a sequel or two and maybe a PSP title.
So for now, I'd have to say nobody. That would be my vote if allowed. Then followed by Sackboy, GT logo then Kratos. However I don't see Sony ever having a character like Mario or as popular that can be utilized as much in different games. Sony doesn't seem interested in marketing a character like that. Crash would have been the closest to that.
I'll come up with something better eventually...
