I think I need 8-10 great games from various systems.
Honestly if I had 1 good to great game every 2-3 months that is re-playable an truly memorable or a classic I am happy.
Though, I would prefer to have something like 20 games in the life time of a system about 5-6 years of those kind of games. I have learned I want less games with story and more games that are just fun to play and easy to play again and again...because story focused games usually don't have the replay I want.
By this standard, I am completely happy with the Gamecube, Wii, and Nintendo 64 lineup. Was highly impressed with the DS lineup and I see the 3DS lineup shaping up nicely.
I think the key is having that diversity in those big hit games though. I only need one killer platformer, one killer FPS, and one killer Puzzle game, racing game ect.