A quilt is not a quilt until it is quilted, and the most important part of that process is the unseen layer in the middle. It’s what makes the quilt warm, makes it hold together, makes it interesting to look at, and makes it a quilt, rather than a coverlet. Without that warm, interesting, constructive, unseen layer, you would be hard-pressed to keep your lovely patchwork for long. You may have a preference for how closely you like to quilt and that will also affect your choice. Check with your supplier for details.
Cotton - it’s not expensive, wears and stitches well, and is easy to find in all sorts of sizes, or by the metre. Oddments can be sewn together for small projects and little gets wasted.
Bamboo - thought by some to be more ecologically sound, but it tends to ‘beard’ through dark colours and make a fluffy mess of the surface when you stitch. Ask for a sample and try for yourself as brands do vary.
80/20 cotton/polyester - is popular and pretty good. It and can be quilted quite openly - that is, with bigger gaps between the stitching lines.
Silk and wool are also available but more costly. Wool hangs well and does not crease so it is ideal for wall hangings. It is also a joy to hand-quilt, like no other.
Polyester is cheap, and it is available everywhere. Some types, particularly the ‘fluffy’ sort, beard badly and do not last well in use. Some of the 'compressed' polyester battings are acceptable for bags and small items.
Curtain interlining or "bump" is used by some quilters for wall hangings but again, ask for a sample before buying large quantities.
For tiny projects, polyester felt is wonderful, and it comes in lots of colours...