I'd suggest you put some thought into what sort of features you're looking for before focusing on opinions of general quality, because what you're wearing is probably going to matter more to you than how well it's constructed or how popular it is.
- tank top or short sleeves?
- baggy or tight fit in the chest/waist?
- coverage around diaper like boxers (covering sides of hips) or bikini?
- step into and pull up (exclusive to tank top?) or pull down with snaps in crotch?
- all white, solid color, or pattern?
- thin or heavy fabric?
I've found I really don't like:
- baggy
- thin fabric (there's one that feels like it's made from a cut up 75 thread count sheet!)
- fabric patterns of any sort
- non-bikini shape at my diaper
There have been some pretty dramatic differences in the onesies I've seen in the last few years. If for example you compared ABU's diapersuit with XP Medical's body stocking, you'd have difficulty finding anything they had in common.
Also keep in mind that onesies are really hard to size. Shirts themselves can be difficult to get a good fit with without trying them on first, and onesies have the added sizing of length and leg width that both need to be pretty accurate to get a good, comfortable fit. Unfortunately they're usually something you have to order online, and can be quite expensive. They tend to shrink quite a bit when first washed also, so watch out for that. Order them a bit large for two reasons: (1) your diaper will add a few inches to the length you need, and (2) you can wash them cold to keep their size OR wash them hot to shrink them if necessary.