Scooby-Doo, Where Are You!, on CBS
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Actually I haven't really seen much of this, but it is a classic, so it needs to be included here. I dunno, maybe I watched some reruns of this when I was a kid, I don't really remember. But I've probably seen a bit of it when I got older, and decided I wasn't really that interested. But like I said, it's a classic, and it is essential for everyone to at least watch enough of it to be passingly familiar with the franchise. And this is probably the best of the various Scooby series, as well as being important as a pop culture reference, particularly in Buffy the Vampire Slayer. Anyway, as to the plot (though I'm sure you're already aware of this), this group of meddling kids (Freddy, Daphne, Shaggy, and Velma) drive around in a van called the Mystery Machine with their talking dog, Scooby-Doo, investigating ghosts and whatnot, and ultimately revealing sinister plots by various folks who would have gotten away with it, if not for them.
There have been several other Scooby-Doo animated TV series that I mostly haven't seen or cared about at all. And there were a couple of live-action movies based on the series, Scooby-Doo and Scooby-Doo 2: Monsters Unleashed. And there's a long-running series of direct-to-video movies (see below). There was also a short TV special called Night of the Living Doo.