I once did a lot of topstitching using Gutermann's then topstitch thread or Gutermann's silk thread that was designed for buttonholes. However back in those far off days I only had a straight stitch machine and I think that makes a difference.
So if you are going to be doing a lot of topstitching in the future it may be well worth buying a straight stitch foot and plate specifically for your current machine unless you have an old straight stitch machine lurking in a cupboard.
I believe the reason most people cannot get a decent topstitch thread is because our machines are built so they can stitch at different widths, some people make an effort to narrow the needle hole on the needleplate by using cellotape.
I must admit I have lost count of the number of dresses, trouser suits that I made that had topstitching on, and they were all perfectly even and the lines so very straight, however I did have a line of contrast tacking thread that I stitched over and then had the job of trying to pull out the trapped tacking threads but it worked.