I'm partial toward Bellino, Anichini, and Frette bedsheets. They are pricey though and retail for over $1000 for a basic set, but can occasionally be found on sale for 50% off.
I didn't read through every post in this thread, but if it wasn't mentioned previously, I've found that true 300-400 thd count is often better than true 500-600 thd count (by true I mean not double woven with artificially inflated counts). The true 500+ count sheets I have tend to be too dense and non-conforming.