As a point of order, I think that the Manta counts as a multi-fx unit; after all, phasers can exist as a stand-alone filter pedal, as can the other variants in there. Plus it has a drive/dirt section.
So, I think you need to either allow multi-fx units (after all, if one were forced to choose only one, why wouldn’t one choose one which covers many bases?) or you need to disallow the Manta.
This is a difficult one for me. The thing is, my pedals function individually as part of a system so having only one part makes little sense on its own. I can safely say that the Mastotron is here to stay, but on its own it is less interesting to me. For sheer versatility it’d be the Manta for me (even though there are better-sounding filters out there).
Honestly though, few things on my board are truly safe.