Window functions with rippleless spectra
That's an interesting question, I personally don't know if, in theory, that's possible or not (would be interesting to investigate). But as a practical matter perhaps you could design a Chebyshev Type II filter, take the IFFT, and use a sufficiently large number of taps. Chebyshev Type II filters have ripple in the stopband but not the passband (although that is monotonically decreasing). I'm not sure how much the truncation would influence the ripple, or if you can handle that large of a FIR. But it might get you close enough depending on your application.

This page details some of the results.
http://iowahills.com/B2PolynomialFIRFilters.html