The Scala Center [1] is the leading not-for-profit sponsor for Scala. Lightbend [2] is the company maintaining the main compiler.

[1] https://scala.epfl.ch/ [2] https://www.lightbend.com/

Not sure if there is a "leading" one ... Lightbend, EPFL, IBM, Verizon, GoldmanSachs, SAP, Twitter, ...