Flop could be subjectively defined in many ways. However, for this particular discussion, let's define a flop as 300 performances or less excluding previews.
There are many, many productions that opened as limited engagements anticipating fewer than 300 performances that were commercial smashes. As CATSNYrevival wrote, a flop is a show that failed to recoup its initial investment costs—it's not really a subjective definition.
But if I had to answer (off the top of my head)... Honeymoon in Vegas.