I assume you want to target the widest audience with your apps and want a good test-bed. In that case you want for your test Linux environments: Ubuntu, Debian, and probably Mint. That will cover probably 90% of the Linux installations out there.
Chiefly Ubuntu and Debian. If you are set up for either/both of those, you are assured that people with other distros can install too. Generally, most distros can work with .deb's (debian repo format) and Ubuntu's repo format.