I agree, Anderson Cooper is the one who was most at ease. I understand he is interested in the job, as is Stephanopoulos, who is yet to host. Ken Jennings has obviously not been doing this all his life, but on the other hand he a ctually knows all this stuff. Richards was good too and I quite liked Aaron Rodgers, demonstrating people skills not always on display in his career. Q&A with Rodgers on the experience here (he wants the job, too):
It's interesting that he mentions this, because so far what seems most obviously challenging for the guest hosts is how to react to someone getting it wrong or those blank moments when nobody knows the answer.