I believe that Fox and Pikachu are our worst matchups. They just outmaneuver us, which is really obnoxious when usually our biggest strength is our mobility. They have safe options on our shield and safe Out of Shield options. Their grab games do a number on us, their dash attacks are fast and excellent, and they are both impossible to gimp.
I think our best matchups are against Fire Emblem characters. Marth/Lucina don't give us much trouble because their range and frame data is even worse than ours, coupled with inferior mobility. Ike is just so slow, so as long as you don't get read like a book, you should be able to dodge him and make his life difficult. Robin is tricky, but has a really hard time keeping Greninja out long enough to set up anything that great. All three of these characters are forced to recover low against us, which makes them prime for gimping and covering options.