It depends on how I intend to use the fabric. I do not pre-wash most of my fabrics for quilts. There is enough stiffness in un-washed fabric for me to accurately cut and piece, so I do not starch that fabric. Depending on the block, I might spray starch the blocks as I go. I usually spray starch an entire top and press before sandwiching. For spray starch, I just use whatever canned starch I find at Walmart or the grocery store. One can lasts me a long time.
For machine applique, however, I like to heavily starch the background fabric yardage before cutting. The easiest way for me to do this is to mix a 1:1 solution of Sta-Flo liquid laundry starch and water. Sta-Flo is readily available to me at Walmart and the grocery store. After mixing the solution, I work on my kitchen island with the yardage. I use a large wall painting brush to saturate the yardage with starch, wait a few minutes to make sure the fibers have a chance to absorb as much starch as possible, then toss the yardage in the dryer. I iron with steam (or spray with water to dampen the fabric) to re-activate the starch. Only then do I cut my background blocks for applique, and I cut them an inch larger all around (and trim back after the applique). This form of starch stiffens the fabric enough so that I do not have to use a stabilizer under the background fabric when I machine applique. Spray starch would not provide enough stiffening power for that.
I also like to heavily starch backing fabric before sandwiching, using the same method in the above paragraph.
Best Press doesn't provide the amount of stiffening I like in a spray starch, so I don't use it. One caution about spray starch -- you need to give it some time to be absorbed by the fabric fibers before you press. Otherwise it stays on the surface and either flakes as you iron or burns if you leave the iron on the starch for too long. The good thing about burned starch is that it washes out later on; it looks as if the fabric is burned, but usually it's just the starch that burned.