Fox's speed is completely sick, agreed. However, I'm basing my argument on M2K's (at his peak) near invincibility with Marth on Final Destination. It is only Mango (also at his peak), using Falco, that I've witnesses Marth being beaten convincingly in this matchup. This is possibly because Mango was able to exploit the flinch-effect of Falco's laser, with Fox doesn't have. Otherwise, people e.g PPMD, have to switch to a Marth ditto.