Sage the hardest Ness match up is the one you concern yourself most with... Personally I find Marth to be hard up to about 60-40 or 65-35. G&W is pretty tough but once you learn the match up you'll be fine... Match up will be about 60-40 G&W.
The reason Marth is difficult is jump break to F air, ground release to smash. Both get me annoyed, I can bare it though. Not landing U airs because of his sword is a minus Ness in this match up. All I can do is abuse my range, and not use attacks without much range. No more d airs to N airs or U airs or even B airs. So this match up really leaves me to 3 very situational aerials and 2 that I have to space well and time properly... Still very playable however
G&W is just funny but his range on his attacks are equal if not slightly better than Ness'. However I'm not as disarmed as I am against Marth. It takes a long time to learn his hit boxes though, that is what makes it difficult.
And sage is right PK flash hits 38.
But in all seriousness, the hardest match up is Bowser... 70-30 Bowser. He has range strength and coolness. Without pk fire and if the bowser is really good you'll have a lot of problems.. His B air can completely danger your recovery with it's strength, his F air is quick and has really long range. His U air will KO you at very early percents. Fire breathe destroys your recovery if used right. Fortress out of shield grab release to powerful tilts and specials. This match up is truly one sided... If you win, it was WiFi or you just got some good pk fire to bats in. That or the Bowser sucks.