Re: Japan/Singapore/Senegal and 2 other passports - in theory a great idea but those 3 countries do not allow dual citizenship.
As far as dual citizenship (with the greatest amount of travel) is concerned, anyone of the G7 country passport (minus Japan) and one other should be fine. Best I could think of is US/Germany/UK/etc. + Chile (visa free throughout S. America, Europe including Russia, India). For visa-free travel to China, if your nationality & business qualifies, you can apply for an APEC business travel card.
As far as dual citizenship (with the greatest amount of travel) is concerned, anyone of the G7 country passport (minus Japan) and one other should be fine. Best I could think of is US/Germany/UK/etc. + Chile (visa free throughout S. America, Europe including Russia, India). For visa-free travel to China, if your nationality & business qualifies, you can apply for an APEC business travel card.