Well, speaking of impactful:
This is one of the things I remember (sometimes horrifically) from my early days on saturday afternoons (after the cartoons and WWE was over!):
https://www.youtube.com/watch?v=dF5oNqw-L3A Channel 48 was one of the 5 or so channels we got (3,6,10,12,17,29, and 48). woo!
Speaking of wrestling, HBO has an Andre the Giant special on which is good. He was and is the man

.
Dr. Who would play on Saturday afternoons also which would run about 2-3hrs or so. I'd watch that before heading out to shovel horse manure (5 bucks/stall i think).
D&D cartoon of course, which seemed to not always be on reliably each week at 11am. Would be hugely disappointed over my honey nut cheerios or Trix if it wasn't on

.
Top sci-fi movies? Gotta think about that one!