Most histories will say that Groucho was in something of a career slump before agreeing to do You Bet Your Life. Make no mistake, Groucho was still one of the most famous stars of his time, the Marx Brothers were still making movies, and the game only reinforced his personal stardom. But the question was looking for that "sweet spot in the middle", and it's not like Groucho went straight from Duck Soup to hosting a quiz. It was definitely "after their career peak".
Although we're too close to it to judge, you could make an argument that Elizabeth Banks is enjoying career success on many fronts (the Charlie's Angels misfire notwithstanding) while hosting a game show currently.