World of Warcraft and all of the expansions to come with it would be ranked pretty well up there. It really is amazing what Blizzard has done to the market and even though it loses my interest, I always seem to come back to it eventually.
The REAL best game purchase for me was the Super NES that I got when I was just a little kid. Why? Because it was my first video game system and honestly, if you think about it, would you be who you are today without ever buying a game console of some kind?