Thomastik Spirocore Viola C String

Best c string. Great sound.

Tungsten C is amazing on a 5-string

My luthier pointed me to these, and they're wonderful. They pack the same or similar mass into a smaller volume (if you've not encountered it, tungsten as dense as gold), so the string doesn't have to vibrate as much, and they reduce the times the C string feels sort of flabby on the shorter scale of a violin. They are about twice as expensive, but my luthier says they last twice as long. I just know they sound great and they're easier and more reliable to play on.