From what I've read and seen – so please take it accordingly – the Ricoh module would surely outperform the R-D1. After all, it's a much more modern sensor with 12 instead of only 6 megapixels, and while the megapixel count is not everything, it still does matter somewhat – especially as 6 MP are really not that much! The R-D1 sensor was also used in the Nikon D70, for example. The Ricoh also has that special microlens layout developed for best performance with rangefinder lenses. This latter feature is missing on the Sony NEX series, as far as I know.
So, in terms of image quality, I would consider the Ricoh and the NEX. (The NEX-5 I have myself but did not use it with rangefinder lenses so far, so I can't really comment on that from first hand experience.)
The Fuji X1 also has the probably same 12 MP sensor of the Ricoh (after all, most of these sensors come from Sony anyway) and the lens is especially designed to work perfectly with that sensor, so the image quality is also very good. That is confirmed by all the samples I have seen so far.
If you really want a true rangefinder, however – R-D1, M8 or M9 – I would definitely rather get a Leica M8 instead of the R-D1. The M8 advantage is not only the much nicer designed body, but also the somewhat bigger sensor that does not crop that much out of the lenses image circle (1.37x crop factor instead of 1.5x). I have some experience with the M8 and love its low ISO quality as well as its handling.
Thomas