I've used the following:
IO - cheap, readily available, mixes well, but the low Calcium was a pain to me.
Kent - most expensive I've used, but typicall mixes up to alk 10dKH, Ca of around 400 and Mg of 1200-1300. My corals like this salt the best and I'm sticking with it. I pay between $40-$65 per 200gal bucket. Price varies WIDELY around here.
Crystal Seas Bioassay - My tank was perfectly healthy on the Kent salt, but it started getting pricey, so I decided to try this salt at $25 for 150gal boxes. BIG mistake. Several of my corals bleached and many of my zoas declined. After about two months of waiting for the bleaching to go away I switched back to Kent. I'll be sticking with Kent unless their formulation becomes inconsistent or different.
Brian