Fox is arguably the best character in the game, but Marth is considered the 3rd or 4th best character, depending on what tier listing you look at.
From what I understand, the top tier is consisting of:
1) Fox
2) Falco
3/4) Sheik/Marth
With Fox and Falco having a slight advantage over Sheik, Sheik having an advantage over Marth, and Marth having a slight advantage over Fox and Falco.
They've redone the tier list with a point system now
Fox and Marth are tied in 1st place which I agree with. Just look at the character of choice who has been used by the champion players. Marth has been top dog. There's no denying Fox's versatility though.
Sheik is in 3rd place with Falco in 4th. I find this to be strange as I feel Falco is probably the 3rd best but in discussing this with my crew, we figured it's because Sheik is one of the few characters that has an advantage on Marth and has excellent matchups with most of the charcters listed below it hence giving it a slight edge with points.