The better match up against marth would be sheik or samus. Link as well. Don't do Y Link as his projectiles are a bit weaker than link's and he's weaker close range as well. Marth can rape IC's in the air so don't do any air attacks with the IC's. Marth also has the grab range advantage. I recommend switching to another character instead.
Sheik for close range ground attacks, watch out for Marth's attacks and don't attempt to juggle him in air, his attacks will outprioritise yours. For Samus, stick to projectiles, bombs and the charge attack and mix it with close range attacks whenever the opportunity arises.
Not sure if DK would be a good counter against Marth as his moves have so much lag but his range is greater than Marth's.