So this is my first impression of 3.6, only confident on the top 5 order and the rest are alphabetical in rough groupings. A lot of characters could go up/down a tier, and I don't even know if I agree with the amount of characters in each tier.
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Basically s tier just has incredible mu spreads that set them above the rest of the cast. I put wolf over fox because while wolf loses the head to heads with fox/falco, I think he wins a lot of MUs harder than fox, and many mus that are closer to even for fox (ddd, marth, maybe rob still?) Wolf should run over. MK is really broken and only loses a handful of mus and dominates a lot of them with just his dd range. Diddy only loses to fox/wolf/samus/peach/wario for the most part, but has a lot of even mus where he has to camp and if he gets touched he dies, which is why I don't think he's better than falco/mk, despite diddy completely destroying all the fatties and slower characters.
The twice high tiers are pretty interchangeable really, just didn't want to put them all in one row. Tons of good, strong, very viable characters that have varying degrees of effort needed to play them.
Last two rows are the actually bad characters. They have moves that just don't work well often, lose to a strategy they have minimal ways around (bowser still loses to the z button, ivy/ness get wrecked by proper cc and sdi, etc). I would have link one tier up but he lost a lot of range on his grab by accident and I think that really really hurts him, because it makes the range that people can sit in shield vs him safely even closer.
DK is just bad and bland.