For most of the period since the Great Depression, most economists have been Keynesian. This is changing to some degree nowadays with conservative economists moving away from Keynesian ideas, but it was true for a long time. Even Milton Friedman, who is not typically seen as a Keynesian, once said that "we are all Keynesians now."
Because everyone was a Keynesian, it is hard to identify specific economists that you would say are major economists identified with Keynesianism. Many of the more famous economists of the last 100 years have been Keynesians. For example, both Gunnar Myrdal and Paul Samuelson were famous economists who believed in Keynes's ideas. Franco Modigliani is known for having extended Keynes's theories and creating models that were meant to help implement those theories. A leading liberal economist today, Paul Krugman, is a Keynesian. So is Joseph Stiglitz.
All of these are names that are connected to Keynesian ideas to some extent and who are relatively well-known, at least in economic circles.