I get a lot of questions about what fabrics I use for my fitted diapers, so I'll go through the fabrics I use, and why I choose them.
I make my fitteds with stretchy materials because I like that they fit longer and seem really comfortable. There are a few drawbacks though. Stretchy materials are a little harder to work with; I use a walking foot and that really helps. The stretchy materials are also usually more expensive than flannel. The non-stretchy materials, like flannel, still make nice fitted diapers though; I do use both, but prefer stretchy. If you decide to use stretchy fabrics then keep in mind that your diaper will only be as stretchy as your least stretchy layer, so don't combine it with flannel (or other non-stretchy material).
These are the fabrics I typically use to make fitted diapers:
Outer layer - I use either cotton interlock or stretch french terry. Both have nice stretch and are a nice weight (not too thin like some jerseys and other knits), and the interlock can be found in cute juvenile prints at Joanns.
Hidden layer - If my husband has old t-shirts to get rid of, then I use those! If not, then I use cotton interlock.
Inner layer - I use cotton velour (it is a blend of 80% cotton/20% poly). This fabric is nice and stretchy, and super soft. It also seems to stay soft wash after wash. I like the cotton velour because not only does it feel soft and comfortable, but it also feels less wet against the skin than other natural fibers.
Other fabric options:
A lot of people like to use fabrics like bamboo fleece or hemp fleece as the hidden layer of their fitted diapers. I do like to do that for nighttime, but I prefer to save those more expensive fabrics for the soakers rather than doing full body layers of them. However, those will certainly work well for fitted diapers, but just be aware that some of these heavier weight fabrics aren't quite as stretchy as the interlock/knit/cotton velour fabrics I mentioned above; so you may have a slightly smaller fit range with these fabrics.
Feel free to comment with any other questions about these fabric choices!