I believe the key of C is an optimal standard. It allows enough string tension to bring out the resonance from the skin.

If you would like a sharp and crispy tone, you can tune up to the key of D.

A lot of modern tsugaru shamisen players will tune to the key of D for it’s energetic brightness. However, being that the strings are stretched tighter, they might break quicker than if they’re tuned to the lower key of C.

If you would like a warmer sound, you can tune to the key of B.

Th key of B gives a vintage tone to the shamisen, as folk/tsugaru shamisen players also kept their string tension low in order to prevent strings from breaking. (You can hear this quality on vintage recordings of tsugaru shamisen players like Shirakawa Gunpachiro.)