The Flabby Patty was a product that rivaled the Krabby Patty. It was only seen in the episode "The Other Patty." They are sold at the Flabby Patty Shack, which was a restaurant made by SpongeBob SquarePants in an attempt to get Mr. Krabs and Plankton to work together. Plankton and Mr. Krabs both wanted to steal the secret formula from the Flabby Patty. At the end of the episode, they use teamwork and successfully steal the Flabby Patty Recipe. They read it and find out it is actually a recipe for friendship. SpongeBob reveals it was all set up so he could make Mr. Krabs and Plankton cooperate. After finding this out, they team up and chase SpongeBob.
It is shown to be better than Krabby Patties.
The Flabby Patty appears to be a rip-off version of the Krabby Patty.
It is possible that SpongeBob put the secret ingredient for Krabby Patties (which makes them so delicious) into a different patty so it tasted better, or that the Flabby Patty is made with his own formula, created using his years of experience as a fry-cook at The Krusty Krab.