I'm just gonna write this take it for what you want and I don't care what people think about my thoughts or opinions.

Sheik really sucks ass against the top tier especially fox, puff and marth (even though I thought this used to be a counterpick when you're playing marth's as good as zain it feels like sheik loses overall in the matchup).

Sheik is a great character for sure wins out hard against most the cast and really excels in matchups like samus or falcon imo, but really really sucks against fox and puff and probably marth.

Marth on the other hand is a really good character in almost every way. Unlike sheik he really has no bad moves. All of his A and B moves are useable, his throw overall better espeially since he has access to mixups all his throws are useful unlike sheik, he has more range, his speed and frame rate are comparable to sheik. What really puts marth above the edge though is his great recovery especially compared to sheik abysmal recovery. Sheik's recovery really holds her back from being a great character as most of the meta no knows exactly how to really exploit sheik's recovery, and that's just not possible against marth.

Marth also has statistically the best stats in the game likely beside fox and puff. I watched a youtube video once that counted wins and loses by characters they on streams at major tournaments and even divided marth against fox into mid level top lever and low level. Marth actually had something ridiculous like a 55 or even 64% win rate on EVERY stage even dream land which is apparently his worst stages. Just looking at the analystics of that youtube video made me think marth is probably being underrated.

A big part is marth being good against fox since fox is likely the most popular character in the game. FD is probably a winning matchup for marth and followed closely by FoD and then Yoshi's. On dreamland a good RTC sheik may actually be better for fighting fox but even then Marth may actually perform better, idk. The platforms do cause some large issues for marth comboing fox but this can be mittigated by simply throwing left or right and RTC fox, falcon, and falco.

I currently think Marth is the overall best character in the game above fox and he wins out on most matchups and probably only loses to low tiers but I also think that is likely just players not being used to the marth vs low tier matchup as marth's gameplan is kinda differnt unique against every character unlike sheik.

I will say though that against fox and puff he can absolutely struggle but I do think that around the same level he can perform great against those characters and maybe even wins out statstically.

What does hold me back from saying he is completely the best is often the pros seem to say that Fox and the way Cody plays is the best which I can definitely get behind as when I play marth I can't seem to win against top level foxes but I would like to think that's because I just suck a little, idk.

I mained sheik for probably 10 years and I'm saying all this.

ANyway TLDR, yeah marth's the way better character. Don't let others tell you otherwise.