When it comes to the heel flap everyone has their personal favourite. I know some people who prefer a plain stocking stitch heel flap but personally I prefer something with a bit more texture and a bit more structure. The plain stocking stitch, whilst being smooth and simple to work can lack structure and end up creating a heel flap that is a bit too loose.
My personal favourite is a slipped stitch heel flap where the right side rows are worked (slip 1, Knit 1) all the way across and the wrong side rows are purled across (after the first stitch is slipped). This creates a thicker fabric as the slipped stitches create an extra layer of bulk across the back of the heel. The slipped stitches draw in the fabric and create a heel which grips better and gives a good fit.
There is also the Eye of Partridge heel where the right side rows are worked as follows:
Row 1: sl1, k1 to end
Row 2: sl1, p to end
Row 3: sl1, (sl1, k1 )to end
Row 4: sl1, p to end
This creates a really lovely texture, but it is a little bit more difficult to remember and for that reason it tends not to be my go-to heel. But it is well worth a go if you are looking for a pretty and fun alternative.