Mario might be the best character in the game and here's why:
In a game where all the top tiers have weaknesses that can't be worked around completely, Mario's are the least pronounced. He's versatile and just SOLID enough and despite some tough matchups (disjoint/kiting characters) I think this could propel him to number 1. He does not have the common issues of closing out stocks, dangerous risk/reward, poor recovery, bad disadvantage, underwhelming advantage, bad neutral, etc. to the extent others do. He really is an all-rounder that bends to the player, not the other way around (the most of any character) However I don't think solo Mario gives you the greatest chance of winning. (Now dual maining Mario/Sonic seems pretty amazing to me). I also still think Sheik and Diddy could claim number 1 as well, because they excel at some things Mario is just great at. Mario is like a 4 star in most categories and they're like anywhere from 2.5-5 stars but some times you need that 5 star attribute.
This is why I think a counterpick meta/dual main meta is STILL the future. No character in this game does everything for you and they've all been designed with specific limitations, including Mario. It would go a ways toward decreasing upsets too- If M2K still had a Rosa would he still have drowned at SSC to that Luigi?
Mario's the best because he's the least likely to be held back by himself. He also has a surprising number of tools available to exploit the other top tiers' shortcomings and, anecdotally, he's the top tier I've seen get exploited the least. (This could easily change in the future if people start abusing Sonic/Cloud/Peach-type characters against him more)
Feel free to tear this post apart because I'm not even sure if I believe it myself but I hope it's offering something new.
Note: I think we have a good group of characters that ARE flexible enough to solo main and succeed with, despite inherent weaknesses, I just think the smartest option, for now, is having a couple of good backup plans. Especially in 2 stock, Bo3.