ok, the top 5 are Fox, Falco, Marth, Shiek, and Peach. Almost all the pros play these characters. If Marth had a disadvantage against Shiek, Fox, and Falco, which make up about 80% of the tournament scene, then probably much less people would play marth. So i feel that Marth must have an advantage against at least one of them.
From what i can tell after reading the fox/falco/shiek/marth forums...
Shiek has a relatively large advantage. Obviously beatable, but the advantage is there.
Fox has a disadvantage.
http://www.smashboards.com/showthread.php?t=58719 According to lots of Foxes, Marth has an advantage on FD, and they are about equal everywhere else, which overall gives marth an advantage.
And Falco SHL = disadvantage, but once the horrible approach is over, you can do pretty well against him.
Therefore i say Shiek = semi counter. Falco = you have a small disadvantage. Fox isnt a counter, in fact you have an advantage.
Ive done a decent amount of studying because i was looking for a good Fox counter because i dont like Jigglypuff vs Fox matchup. Marth and falco were both viable, but i dont like falco very much :-P