6. Amy Poehler as Granny:
Episode: "Have You Seen This Snail?"
7. Patton Oswalt as Jim the Fry Cook:
Episode: "The Original Fry Cook"
8. Johnny Knoxville as Johnny Krill:
Episode: "Extreme Spots"
9. Dee Snider as Angry Jack:
Episode: "Shell Shocked"
10. David Bowie as Lord Royal Highness:
Episode: "Atlantis SquarePantis"
11. Ginnifer Goodwin as the Teenage Mermaid:
Episode: "Welcome to the Bikini Bottom Triangle"
12. Mark Hamill as Moth:
Episode: "Night Light"
13. Alton Brown as Nicholas Withers:
Episode: "House Fancy"
14. Chris Elliot as Lord Poltergeist
Episode: "Ghoul Fools"
15. Biz Markie as Kenny the Cat:
Episode: "Kenny the Cat"
16. Amy Sedaris as Ma Angler:
Episode: "Trenchbillies"
17. Michael McKean as Captain Frostymug:
Episode: "License to Milkshake"
18. Gene Simmons as Sea Monster:
Episode: "20,000 Patties Under the Sea"
19. Johnny Depp as Jack Kahuna Laguna:
Episode: "SpongeBob vs. The Big One"
Via this Wikipedia page that lists all of the show's guest actors.