Again with the young people comment.
I think this translates into English as the person with whom you have fun. Or something.
So that all depends upon the context: most fun on holiday might not be the same as the most fun at work (yes, I do have fun at work!) or the most fun shopping or the most fun travelling. Different people are best companions in different circumstances. Thank goodness.
Think of it like matching your accessories to your outfit.
So I am going to pick the most fun at work. Because I can. And because I have a good photo for it.
Thank you all. And please note, this does not mean I think you ever messed up!