Gamerhelp has produced a list of 18 genre-defining 'pinnacle' games. I'm not sure that defining a genre and being the highest achievement in that genre are quite the same thing.
As a case in point, the best Adventure is Quest for Glory I. Now, I love the Quest for Glory series (I named it my favourite in this very thread), but how did it define the adventure genre when it's an adventure-RPG hybrid, and perhaps the only true adventure-RPG hybrid?
I also notice that Fallout 3 is declared a 'worthy heir' to Fallout, impressive since it hasn't even been finished yet.