This is what I said in a PM to someone that asked:
I have the benefit of playing ~2000 games, almost exclusively with customs, and without a main. (I play ~20 characters, which for the record is a terrible idea.)
If I were to rate the extent to which characters benefit from customs 1-10, strictly based on my personal experience and current opinions:
(Keep in mind, this is considering how much the character's overall matchups improve. If a character has a great custom, but an even better default, it doesn't matter. Meanwhile, if the character has a "bad" custom that is still worth taking in certain matchups (WFT and Little Mac come to mind), they do benefit some. Rosalina's placing also factors in that a lot of characters Luma used to faceroll gain access to anti-Luma tools.)
I'm not including



because I have zero experience
without customs when it comes to them, to compare to.
10 -

9 -


8 -



7 -




6 -





5 -








4 -








3 -









2 -






1 -
Filtered for no-customs top 50% only, according to me:
10 -
9 -
8 -
7 -

6 -


5 -





4 -





3 -





2 -





1 -
Filtered for no-customs bottom 50% only, according to me:
10 -

9 -


8 -



7 -



6 -



5 -



4 -



3 -




2 -

1 -
One final tidbit: I ran the numbers, and imo just over
60% of default specials are actually best-in-slot. They seem to have been very conservative in making the customs.