The best archive format is ePub. It is the richest of the commonly available reflowable formats, and is starting to be available on dedicated reading devices beyond the Sony Readers. LRF is a dead end, because it is never gong to be popular beyond the Sony's.
However, I'm not sure I would delete the originals. This is because conversion to ePub may improve over time. I would expect MOBI to ePub and LIT to ePub to perhaps already be optimal (although I am not certain this is the case), but existing PDF to ePub conversion probably can be improved on. On the other hand, if you manually update the ePubs then they can be better than the originals. I don't know how well LRF to ePub conversion works.
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