I say there could be numerous definition to the subject heading but the specific criteria here is that if the game was released tommorrow, it could manage to sell more than one million copies first week in a single territory.
My top ten would be:
1.GTA
2. Pokemon
3.Gran Turismo
4.Super Smash Bros.
5.Mario Kart
6.Halo
7.Final Fantasy
8.Dragon Quest
9.Monster Hunter
10. Super Mario.
Edit-Oh now, I wish you can edit the title. Think, not thing.