New there`s the CV 90mm F3.5 APO Lanthar which is Stunning wide open though can be tricky to focus like all 90s and longer due to the short Rangefinder base on the R-D1 - it`s beautifully made, doesn`t vig and is tack sharp at all apertures..... Doesn`t break the bank either..
The non collapsible Leica Elmar F4 mentioned above could be a good buy used so long as you can find an example which hasn`t gone milky at the right price and of course the black M mount version for the Leica CL (dunno how they focus though).. I compared the old Elmar to the CV 90 on thursday and the CV won, especially at the edges but samples do vary due to optical contamination, balsam separation etc.
Then there`s the above mentioned Jupiter-9, the cheapest option (though prices are rising fast - it`s not a £5 lens like the 50mm Jupiter-8) ..
I`d say that a new CV 90mm Lanthar is the safest bet, though not the cheapest.