Are we choosing a mascot based upon the current market or are we asking based upon pre-existing "mascot" or otherwise iconic characters? If it's the latter, it's a very limited field, but if it is, you really can't beat Mario for sheer notoriety and longevity.
If we are talking about choosing a mascot from various characters that could represent gaming in general, then it becomes a bit more of a debate, since like someone said, Mario has kind of lost his relevance in the change in game content and the shift in audience.
I'd have to go with Solid Snake. The Metal Gear games are mixture of straight-up action fighting, strategy and thinking, and puzzle-solving, widely representative of games today. The deep (even if overly detailed) plot deepens the role that you enter into as Snake. He's a hero and a legend, if not a little damaged and imperfect, not unlike humans in general. Regardless of what you may think of Kojima's Metal Gear series, there's something to be said about the longevity of Solid Snake and the type of gameplay he represents, something definitely more mature than Mario could ever be, but still not associated with something socially controversial or objectified, like a Lara Croft or a Nico, or even Master Chief.
In my opinion, if you want a representative for games these days, it's Snake.