Best actor in Ken Park...hint: It wasn't Claude...
Claude was a good actor--actually, several were regardless if one agrees with the quality or content of the source material. But I'd argue that the best actor was the brief non-top billed appearance by Claude's friend during the group pot-smoking scene. Their interplay was nicely-acted and entertaining as they exchanged conversational back and forth.
But it was Claude's friend who charismatically upstaged the other actors in this movie, even though he only appeared briefly in this one scene to share an emotional story about his comatose father. In his short time onscreen, he revealed perhaps the sole redeeming trait about deeply-failing parents, in that they're simply there. It felt so real and unscripted that I suspect even the most victimized and traumatized people watching it would consider other options beyond the final one the character Ken Park took.