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.