Second Reality was the best. First demo that I saw was Spaceballs' State of the art https://www.youtube.com/watch?v=c5kuYfTCGLg and I vividly remember of thinking 'wtf is that, a video or what?' and then Second Reality a year later or so - https://www.youtube.com/watch?v=rFv7mHTf0nA But then (I look all and every demo out there by then at the age of 13) I saw Mars demo https://www.youtube.com/watch?v=_zSjpIyMt0k which hooked me forever on the path of graphics programming.

Oh yes! I remember Mars! At some point, the was a text floating around the net which described how it worked, that was mind-boggling for the 14 year old me.