Stagewise, I'd take him to either Final Destination, Brinstar, or Rainbow Cruise. In the air, he's not that threatening, it's only really if he lands an FSmash or an USmash that a KO is truly likely. Sure, he has other options, but these are the only two that really threaten Mario as finishers. While Mario does have more kill options in this matchup, Fox has way more to rack up damage with, not to mention he's quick enough to make fireballs an ineffective tactic against him in comparison with B-Air WOP'ing. I'd say this matchup is even, and I think it's the most exciting matchup for Mario, because both players gain little-to-nothing from being overly defensive in this matchup, so, at least from when I've played them, the more aggressive player usually wins.