From what I can gather, Rumia's the best option since she's the embodiment of "Darkness" that will always exist along side light so she'll always come back. Clownpiece...idk.

This was a weird one for sure.

Basically the story is that in this Reimu is a physical representation of the Gensokyo itself and in order to maintain it's power she needs to consume something, ie. the "infinite" darkness of Rumia.

Yeah, it's a weird one for sure but interesting.

Also I think Clownpiece will come back, she was purified by Junko those reaching an "infinite" madness so to speak.

Fairies normally come back when they're killed, too, as they're aspects of nature. I'm not too sure if that would work with the more spiritual eating, but perhaps.