There comes a pivotal moment in every person's life where you can't help but pause for a moment, and think, "you know, there really needs to be another fun fact this week!" haha. Given it's Don Knotts related and this is Don Knotts Week, I mean, how could I Knott take this opportunity?
Out of all the guest stars in Scooby-Doo, all the various Don Knotts look-a-likes that appear in various Scooby films and movies would technically put Don Knotts over the edge for the guest star that has made the most appearances in Scooby.
Besides his main three appearances in "Guess Who's Knott Coming to Dinner," "The Spooky Fog" and Night of 100 Frights, characters that look or act like Don Knotts appear in the following episodes and films:
Don Knotts also appears under one of the masks in "Bravo Dooby Doo."
I'm admittedly a little surprised they didn't get Don to voice Mr. Claphammer, given they had just gotten Don to voice the groundskeeper in Night of 100 Frights a few months prior. I thought it could have possibly been due to Don's health issues at the time, but he voiced himself in Johnny Bravo two years later, so I don't think that was the issue. Maybe the character was just a parody of him. On an amusing note though, when looking up when Don passed away, one of the auto-suggested searches was "Did Don Knotts and Andy Griffith die at the exact same time?" haha. That would be super intense if that had happened lol.