For me it was Daniel. Kreese was really the source of Daniel's torment in 1 and 3, and he never got to get Kreese back. Johnny stuck it to him by calling him out at the beginning of 2. That stung for Kreese. Nothing Daniel ever did directly ever stung him. I mean yeah, winning the tournament was tough, but what matters to Kreese is what is done directly to him. This was Daniel beating him directly, and that felt as satisfying as watching Luke in the Mando season finale.