Take the title as you will - "most disappointing" or "most underwhelming" or whatever.
My personal least favorite Cave shooter is... Fever SOS (didn't play Feveron, so I won't call it by this name).
Besides the groovy soundtrack (the boss theme's sirens are annoying, though), I didn't find anything special about it.
Spoiler
Q: Why "least favorite" and not "worst"?
A: Because I think "best" and "worst" are objective terms, and I do not want those. I want personal opinion. And there is no universally hated Cave shmup. I think.
Q: Are you stupid? Do you want to start a war and get banned as a result?
A: Maybe.