By onesies do you mean a t-shirt with buttons at the crotch, a romper, a union suit, or footed PJs? It seems all of them are called “onesies” on the market... I’m assuming you mean just the first and for males (given the name of Link)...
Hands down, the most comfortable cotton onesies are the Tykables, without question. Highest grade of material and workmanship. I like these more than I like a lot of my non-ABDL cloths. They are just super comfortable!!! I also give points to the ABU ones for having the best elastic around the legs to properly secure the diaper.
Least favorite are LittleForBig, AwwSoCute, and similar. These are paper-thin. Also, these brands, like many others on the market are actually just women’s bodysuits. They don’t properly fit males at all, and aren’t cut as onesies for supporting diapers.