Hmm, that's hard. I actually haven't fought any of them lately. I haven't fought Inui and Spam in FOREVER, and I've gotten LOTS better at the match-up since then. I fought M2k's MK a few weeks ago, and I took him to last stock last hit. Basically, when fighting M2k, just don't ever shield or dodge, and you'll do fine. He just waits for this, and then ***** you in the opening after. M2k's whole style revolves around abusing the openings you create yourself. Recklessly attacking him and challanging everything helped me discover just how powerful Marth can be on offense.
Aside from Umbra, the last MK I fought was Forte. I lost, but since then, I've become considerably better at the match-up, since my brother has finally become a formidable MK.
Seriously, I don't think the match-up is that bad anymore. While on the defense, MK can pressure Marth, and the problem is that Marth players are intimidated by MK, and so tend to overly rely on defensive options like shielding and dodging. MK is too quick for these options to be effective, as he can easily hit us out of dodges quite reliably, and doesn't have enough lag for the shield to be a super-powerful tool vs. him. Marth's sword on the other hand, outranges MK's. So long as Marth applies the proper pressure, and DI's correctly, he can fight MK on nearly even footing. If you'd like some examples, I'll see if I can have Steel upload some videos of me vs. Umbra.