This is another tough one! I would honestly have to go with "Wednesday Is Missing" through "Sandy Duncan's Jekyll and Hydes," as I believe any other five-episode stretch would have an episode I don't love or just think is okay in it. I do love "The Secret of Shark Island" though, I just like "Wednesday Is Missing" a little better!
I might be a bit lower on A Good Medium is Rare and Sandy Duncan’s Jekyll and Hyde. My rankings went as followed
1. Lochness Mess
2. Dynamic Scooby Doo Affair
3. Ghost of Bigfoot
4. Secret of Shark Island
5. Haunted Horseman of Hagglethorn Hall
6. Phantom of the Country Music Hall
7. Frickert Fracas
8. Haunted Carnival
9. Haunted Showboat
10. Guess Who’s Knott Coming to Dinner
11 Mystery on Haunted Island
12. Caped Crusader Caper
13. Weird Winds of Winona
14. Ghosty Creep From the Deep
15. Haunted Candy Factory
16. Wednesday is Missing
17. Ghastly Ghost Town
18. The Exterminator
19. Spooky Fog
20. Sandy Duncan’s Jekyll and Hyde
21. Spirited Spooked Sports Show
22. A Good Medium is Rare
23. Ghost of the Red Baron
24. Mystery in Persia