Marner makes the most sense to move. Tavares and Matthews are here to stay, and I believe that Nylander provides the better value on his contract compared to Mitch.
Having 3 forwards making $11 million is already crazy to start with. He's a great playmaker but goalscoring is always more coveted and the rarer skillset, and it's something Matthews, Tavares, and Nylander can all do better. The latter three can also all play centre, a more coveted position than winger.
Marner would also bring back the most in a trade compared to Nylander. Could get an actual top pairing D-man one for one and would likely still have dollars leftover to address an area of need.
Marner has also probably rubbed the organization the wrong way the most. His contract negotiation as far as I'm concerned was the dirtiest of the three. Nylander took till the final minute yes, but his camp never ran to the media stirring up controversy and drama. Nylander's contract is also looking like the best of the three. Nylander for all the work ethic criticism thrown his way has scored a ton of his goals from the front of the net, and has shown a marked improvement in his game. Matthews has as well, while Mitch seems to have stagnated.
The biggest reason to move him would be the message it would send. The hometown boy, drafted here and signed long term gets traded sends the message that no one is safe and that management is not satisfied with anything less than excellence. It would be a real wakeup call for this team that seems to have developed it's own version of blue and white disease.
Mitch is still insanely talented and who knows, maybe they do grow up and figure it all out. But as it stands right now this team as it's currently constructed can't and won't compete for anything substantial.
Deal Marner, and go forward with a core of Matthews, Tavares, Nylander, Rielly, Muzzin, New top pairing D-man, and Andersen.