So here are my thoughts, not quite in a full list though I could make one.
Customs on or off makes a huge difference. Customs on is way, way better balance.
I see top 4 as Diddy, Lucario, Sonic, and Rosalina in that order with customs off. Customs off Diddy belongs in his own tier at the top and is just distinctly non-trivially better than every other character; customs on I suspect Lucario is probably #1 by a small margin (it's really close but Snaring Aura Sphere is the sort of thing that especially helps against characters like Diddy and Sonic up here among the tops) and Rosalina/Sonic are relatively closer to the two ahead of them (I really want Luma Warp vs Diddy so bad...) but I don't think the order changes. Villager also enters the picture with Timber Counter and Exploding Balloon Trip to make top 4 more like top 5 (he probably takes #5 with customs on but belongs with the top N characters instead of in also ran land); with custom off he's right near the middle of the cast and won't prove a notable character in the long run though he's not bad.
Customs off I'd probably put the race for #5 as between Yoshi and Sheik; I feel like both are just not *quite* there with the tops but are really close and definitely fully competitive. Customs on it's so chaotic between so many characters for #6 I don't even know; I feel like you could just name characters at random for #6 for customs on and have a good shot of saying something smart because it compresses the tiers so much and all of the previously bad characters who are suddenly good change the match-up calculus enough to even throw characters like Ness who might not have benefited so much directly from customs into the argument again.
A lot of these lists are seriously sleeping on Ike and Shulk. Customs on helps both a lot (ESPECIALLY Ike who I'm pretty sure is in the fight for the highest tiers in customs on), but even customs off both are legitimately good characters and no worse than mid. Everyone talks about Marth being not so bad (and I agree he's not), but I'm pretty sure if you want a sword character both of these guys are better than Marth.
Customs on makes Samus, Ganon, Charizard, Palutena crazy better. These characters are all legit with customs on but just garbage with them off; they're the most stark balance cases of how much customization matters.
Greninja doesn't belong so high on lists; he's the worst character of his archetype. Diddy, Sheik, Zero Suit Samus, Sonic are all in the game as speed characters and all better than him (well, Sonic isn't really that similar, but he is better for what that's worth). This statement holds with either customs on or off; I think Greninja is overrated.
I'll risk saying it; Pac-Man is awful. Customs on helps him since he gets some really good options (On-Fire Hydrant is ridiculous!), but he's still a low tier character from where I'm sitting either way (customs is just a question of whether low tier has viability, not of whether he's in it). He's just so unsafe as a character, and he lacks solid goto options or really anything oppressive to do to assert his dominance. He's not even really a power character; for how unsafe so much of his stuff is, he should probably hit as hard as Bowser, but he doesn't. I also foresee him getting worse as the metagame advances; if he ever got to be a real threat in the metagame, people would just practice catching his fruit, and that would be a really big problem for him since it's not like he has any kind of a real rushdown game (unsafe attacks AND the worst grab in the game, great combo) so if you as a player had no fear at all of camping him I dunno what he would do.
Wii Fit Trainer is not bottom tier. Her range on a lot of stuff is awkward, but her speed-power ratio and projectile game are both great. To be fair, I mostly think of her in the context of customs and she benefits more than almost anyone else. Jumbo Hoop gives her a solid hitbox option and changes the game for her. She's probably low tier (but not bottom) in customs off, but I think she's a legitimately good character in customs on (who also has bar none the most diversity in playable load-outs).
Mii Gunner is IMO the worst character either way. The character just has poor options to finish stocks and awful options to get out of disadvantaged situations. If everyone played Bowser and Ganon Mii Gunner might be okay, but I honestly don't know what this character is supposed to do other than lose against characters who don't care about his projectile zoning. I have long felt Mii Swordfighter is underrated but probably still lowish. If I'm just talking about Miis, Mii Brawler is transparently at least decent, but I'm not convinced he's anything special.
Speaking of projectiles, am I the only one who increasingly gets a sense Duck Hunt may not actually be all that good? His control is really great and it's hard to write off a character with his kind of options, but his "get off me" is really poor (I haven't looked into his frame data, but it feels really bad in gameplay) and this character struggles so much to finish stocks which can just create all kinds of problems for him especially if he starts losing and needs to make a comeback. He feels like a less extreme version of Snake except with bad kill moves instead of very good ones, and that is making it harder and harder to see him as high tier.
I have no idea where Peach and Jigglypuff fall but heavily suspect both are being very slept on. Both need to see *way* more play.
If it sounds like everyone moves up with customs on it kinda is because they do; the tiers mostly just compress with a few characters I mentioned shooting up relatively. That's my balance impression, and while I could talk about other characters, these are the main stand-outs from my pondering of the balance. Given that I see it how I do, it is probably not a wonder that I'm such a customs advocate; it helps the game a lot!