Marner would be their best player, and lets not lie to ourselves about it. I know its popular to rag on him and diminish his skills/abilities.
Aho is a better scorer than Marner, and he plays C, but, Marner is still better.
Marner on the Canes is still a 95+ point player, while Aho on the Leafs may even have his production go down, as he'd not get as many looks.
Personally, i think we keep Marner, and i'd love to see him moved to C. If smaller players like Aho and Bedard can play C, it should be easy for Marner, since he was often playing as a center before drafted, and he's solid defensively.