I can't do any less than these 30, and I know I had to cut some really good ones out to make room (Turok: Evolutions and America's Army 1.2 Overwatch, I'm sorry...).
I'm not about to justify these choices to anybody; I don't consider this a list of the best games/designs ever. This is just a list of the games that I've enjoyed playing the most/have had the greatest impact on my lifetime. I've left them in alphabetical order, as I simply don't think I can put any one of them above the others.