The Henley Passport Index rates all the different countries’ passports based on visa-free travel. From their website “With the Henley Passport Index, you can assess where you lie on the spectrum of global mobility and discover how to maximize your travel freedom through citizenship-by-investment”. Japan and Singapore are currently top of the list.
Interestingly enough, I believe that Japan has recently made it a lot easier to get residency over there. From this site about Japanese Residency: “If you have been approved as a Highly Skilled Professional (HSP), you may apply for Permanent Residency after 3 years from the date you were approved as an HSP. If your application for an HSP No.1 is assessed as having 80 points or more, you can apply for Permanent Residency in 1 year”. Although obviously that is residency and does not give you a Japanese Passport.