Fox vs MK/Marth/G&W: These are all disadvantages due to Fox's inability to deal with zoning at a mid range. If Fox's opponent remains a distance such that Fox cannot safely laser, yet far enough away that they can throw out walls, Fox becomes limited. Fox doesn't have the priority needed to deal with moves such as G&W's b-air. Fox's best option is to dash away and then dash back in during any cool down or lag the opponent may have rather than attempt to challenge them in priority. However these characters all have frame traps to zone Fox. Fox's fresh upsmash will kill at the following percents for them when correctly DIed on neutral stages (Battlefield, FD, Smashville, etc... all have near-equal ceilings): MK: 95%, G&W: 92%, Marth: 102%. If an opponent starts to play definsive with zoning near these percents, then trade off with up-smash. Example: You can trade G&W's bair with Upsmash if you space correctly (which is hard if G&W is spacing you as well). Fox has lots of risks in these match-ups. I'm leaning towards MK 30:70, Marth & G&W 35:65 at the moment.
Pikachu, Sheik, and ZSS: Fox's worst 3 match-ups. All have the best capabilities to exploit Fox's falling speed.
Pikachu: At 0% Pikachu can chainthrow Fox. Many people downthrow with Pikachu into an upsmash resulting in around 84% if done correctly. However, what's an even better exploitation is slowly moving towards the edge with the chainthrow, and at the edge ending the chainthrow with a n-air. This sets up Fox for a kill rather than upsmash which does not set up for a kill unless Fox poorly DIs and lets himself get hit by thunder. Overall, it's a glaring disadvantage and Fox does not have the options to avoid being grabbed by Pikachu.
ZSS: I'd say this is the easiest of the three. At 0% ZSS's downsmash can be used to combo Fox 0-death legitmately. ZSS can continue the downsmash chain until a percentage where bair will kill. However, what makes this match-up the easiest, is the fact Fox does have the ability to avoid ZSS's downsmash. Fox can manevour around ZSS much better than she can manevour around you and set up a downsmash. However if she does land a downsmash below 35%, it's a stock, so it's a fairly disadvantage position for Fox and he does have to limit himself some to avoid it.
Sheik: At 0%, Sheik's forward tilt can be DIed down and you can sheild before Sheik can continue the lock. At around 35% Sheik can then initiate the f-tilt lock. If both Fox and Sheik do their parts perfectly, Fox can escape at about 70%. The best option for Sheik is to hit Fox with an up-tilt at this time, putting Fox at a percent where she can then do 1 f-tilt to tippered up-smash for a kill. So basically Sheik's plan of action is to get 35%, then land a forward tilt and combo, then land a second f-tilt and kill. Also, Sheik can edgeguard and gimp Fox just as well as Fox can do to her.
Upsmash Kill percents: Pikachu: 98% Sheik: 97% ZSS: 98%
I'd but the match-ups at: Pika: 20:80, Sheik: 25:75:, ZSS: 30:70