JK Simmons is a great actor because he has a talent for using his voice and body in a way that convinces the viewer that is in fact the character he is playing. He appears to throw himself into character with passion, such as working out to get huge muscles for an upcoming role, and it is a pleasure to watch him.
JK Simmons has a long and varied resume of acting positions. His roles have varied between dramatic and comedic and he has even ventured into voice-over work. Any actor who can successfully play both a dramatic role and a comedic role should be considered a great actor. JK Simmons has avoided be type-cast but showing dimension and depth in all his characters. He commits to each role he takes on and supports the other actors in each production.
JK Simmons has a supporting roles in a variety of films as well as popular tv shows. His voice also is prevalent through the voiceovers he has done. Having said that I think he will continue to have a lot of supporting roles, but not leading roles. I'm not sure what he made for doing those ridiculous (but not in a good way) Farmers Insurance commercials, but I do think that demoted him as an actor. When actors pick up roles/commercials gigs they should choose carefully. Just like Jennifer Aniston seems to be devalued over her (high paying, I'm sure) United Emirates commercial, I don't think JK Simmons is going to be cast in any Oscar-potential leading roles since he's now seen as a bit of a nutty professor.
JK Simmons is a very good actor, but not one that I would classify as great. However, one person's good is another person's great, and vice versa. No doubt that Simmons is talented, but his roles seem to all fit within the same genre. He seems to be typecast to a particular role. Greatness covers a wide array of talents.