Could be because when we experience a rewarding moment, we take note of what brought it about and try to do it again and again, so it becomes hard to say which one was the best or most rewarding.
Or, it may need to be narrowed down a little, like: what’s your greatest personal accomplishment or what’s the best reaction you’ve had to a self-less act….(of course that will raise the question of there being such a thing as a self-less act if you relish in the reward of it…)