To name the top tiers that can go even with MK:
Diddy - bananas screw Mk over just as much as any other character. diddy's fair also beats mk's anything.
Olimar - this match up is basically a marth ditto. Who ever gets the first hit in is winning. MK juggles Olimar forever and gimps really well, but Olimar racks up damage crazy fast and kills mad early.
Marth - If you make solid reads you can actually overwhelm MK. It's hilarious, because MKs are so used to ****** every character, then marth comes out of no where and shows MK how it's done. Also, grab nearish to the edge = very early stock. kills MK at around 70%. MK can still be MK and **** marth though.
Pikachu - Pikachu has really good anti-air options and can solidly out camp MK. Plus, if you're esam, you can infinite him.
Ice climbers - Grab 3 times, win.