New Matter filament of course!
However, If you decide to use a different brand of filament we do recommend that you look for highly reviewed and often reviewed 100% PLA filament with your pen. Not all filament is processed equally so purchasing lower quality filament will have a significant impact on your enjoyment of the Quil. It will at best lead to design quality you're not happy with and at worse cause issues with globbing or clogging inside the device.
We do recommend steering clear of hybrid filaments that may include wood particles or metal flakes as it may cause an irreparable clog