It is tough to narrow it to 15 with all the great games out there. Here are my top 15.
Gabriel Knight 2 The Beast Within
The Longest Journey
Secret of Monkey Island MI1
LeChucks Revenge MI2
Curse of Monkey Island MI3
Simon the Sorcerer 1
Simon the Sorcerer 2
Day of the Tenacle Maniac Mansion 2
Broken Sword 1 Circle of Blood
Legend of Kyrandia Book One
Zac McKracken and the Alien Mindbenders
Honorable Mention to:
Black Mirror,Sanitarium,Kings Quest 7, Tony Tough, and the Space Quest series.