When I was younger, I managed to get my hands on a copy of Scooby-Doo Mysteryfor the Sega Genesis. It was a standard "point and click" style adventure game that let you play through two different settings, a hotel belonging to Daphne's...uncle, I think, and a carnival. One was haunted by a Native American chieftain an the other by a demented clown. You can probably figure out which was which.
The graphics were simple, and it had a very strong sense of the days of adventure games that included Sam and Max: Freelance Police, Day of the Tentacle, Full Throttle, and other great games made by Lucasarts.
It was fun, and it managed to capture the investigative nature of the series quite well.
My sister recently has started playing some of the newer Scooby-Doo games for the Wii and Gamecube, and having glanced over now and again to see what they're like, I've come to the conclusion that they are the absolute worst things to ever happen to Scooby-Doo.
And yes, I'm including that series where Shaggy inherits millions of dollars and Scooby Snacks give the dog special powers.