Only one I've used is Global Herbs Instant Supercalm for a farrier visit. Belle was much more sensible but I can't be certain whether it was the calmer or just the fact that she had settled in more... I imagine a combination of both. I'll use it again next time and I have heard good things about it in regards to use for shows, travelling etc.
I guess it depends what you want calming for. I have used sedalin, with a horse on box rest who needed some serious calming to wash off feet & tail whilst in/first time turning out etc, but obviously that is for a singular event. Longer term, I have heard good results about horse first, relax me, but I haven't used it myself.