It all depends on the person, what are their tastes. If they don't like what I like then it could put them off but I'll have a go.
I would avoid any crossover, period. IMO few rise above average with most hovering around the meh category with some like Speed Demon being dire. Yes there are a few good ones like Brave and the Bold and Gourmet Ghost which might be ok, but as you have yet to establish any of the characters you're putting the cart before the horse.
I would avoid the live action movies, as with the crossovers you have yet to establish the characters. Plus they all tend to be a bit naff as well.
Things I would consider LEGO Scooby-Doo: Knight Time Terror Camp Scare Big Top Shorts like Beach Beastie and Haunted Holidays A couple of episodes from SDWAY, TSDS and WNSD
Once they have an idea of the characters and the general themes then just say that there's a pile of other stuff and refer them to this site and their episode list.
I agree with Russm's sentiment about it depending on the person and their specific tastes. But generally,
Where Are You, obviously, maybe "A Night of Fright is No Delight" or a couple of the early ones What's New, Scooby-Doo? Zombie Island / Witch's Ghost Boo Brothers / Ghoul School The New Scooby-Doo Movies - specifically "The Loch Ness Mess" (though I am biased with that one)
If the person in question prefers the "man in a mask" aspect, then "What's New, Scooby-Doo?", the associated movies (Loch Ness Monster, Aloha, Where's My Mummy, Pirates Ahoy, Chill Out, Goblin King and Samurai Sword), "A Pup Named Scooby-Doo" and the classic "Scooby-Doo, Where Are You?" and "The Scooby-Doo Show" series.
If the person in question prefers the "monster is real" aspect, then the three 80's movies (Boo Brothers, Ghoul School, Reluctant Werewolf), the revival movies (Zombie Island, Witch's Ghost, Alien Invaders, Cyber Chase), "Scooby-Doo! Mystery Incorporated", "The 13 Ghosts of Scooby-Doo", "Scooby-Doo" (2002), "Scooby-Doo 2: Monsters Unleashed", "The Scooby-Doo Project", and the "Scooby Apocalypse" comic.
all would depend on what era of Scooby that they might be interested in. after all scoobys been around for 50 years, and there been more episodes of scooby, in his various forms than any other cartoon in the Us ever , save for the Simpsons. well over 400.