Interestingly, Gabe's favorite non Valve game is Super Mario 64.
http://www.nintendolife.com/news/2014/03/valves_gabe_newell_thinks_super_mario_64_is_just_swell
http://www.nintendolife.com/news/2014/03/valves_gabe_newell_thinks_super_mario_64_is_just_swell