In your opinion: mystery inc./the gang's FIRST encounter with real monsters and the supernatural was:
"Ghost Who's Coming to Dinner?"
"A Halloween Hassle at Dracula's Castle"
Scooby-Doo on Zombie Island
Nibiru and the Evil Entity
Scooby-Doo and the Goblin King*
"The Ghosts of the Ancient Astronauts"
Straight Outta Nowhere: Scooby-Doo Meets Courage the Cowardly Dog
*even if Fred, Velma, and Daphne forgot by the end
Other notes
The 13 Ghosts not included because of the absence of Fred and Velma. If you think this is the first encounter for Daphne, Scooby, and Shaggy let me know in the comments
ditto the above for "Peephole Pandemonium"
Also, while there's a handful of episodes from The New Scrappy Show/New Scooby-Doo Mysteries where Daphne, Scooby, and Shaggy (w/ Scrappy) encounter real monsters/supernatural, Daphne is not aware or reasons it away
Shaggy and Scooby have too many encounters to list
Not counting "Reef Grief" because that iirc was a natural biological evolution
Not including "Scoobynatural" as it was a SPN episode not a Scooby one
None of the movies after Zombie Island count because I assume they happen after and therefore would not be the first encounter
If I have forgotten or missed any encounter including the 5 original members let me know