These episodes both introduce important things to the series. Although, I think Plankton! is much better than Jellyfishing, plotwise and humorwise.
Jellyfishing: When Squidward ends up getting stung by a jellyfish, he has to wear a full body cast and use a wheelchair. Spongebob and Patrick decide to make it up to him, by throwing his Best Day Ever!!!!!!!!!!!
There are a few funny parts, like when Spongebob tries to feed Squidward soup, or the "Firmly Grasp it!" scene. Overall Grade: 78%/C+
Plankton!: This episode is much funnier and much better than Jellyfishing. Plankton goes inside Spongebob's brain to the get the Krabby Patty formula.
I love The Letters of the Alphabet song, Plankton's lab, and labratory, and of course this scene
Spongebob: Shut your mouth, you mediocre clarinet player.