Originally posted by aaacb Fa 43 for me, I like it a lot with manual focus cameras, fully mechanical or not.
This or a Voigtlander Ultron SLII. As you are going to Japan you might pick either up much easier there than you can elsewhere, so maybe a M 40mm f2.8 until you get there!
This will be a trip of a life time, I guess, so do not skimp, whatever you chose to take. 40-45mm is the best general focal length for general photos as it most closely matches ones eyes FOV.
Personally if I were going on your trip I would take the following :- M 20mm f4, M 40mm f2.8 and M 85 f2, to have all bases covered. But if restricted to only one it would be the M 40mm f2.8, unless I were fortunate enough to own either the Voigtlander or Ltd 43mm. Oh! I almost forgot, plenty of your favourite film stock!