i asked this question once too. seachem,reef crystals, and instant ocean surfaced the most.
I went with sea chem, it doesn't dissolve as nicely as the others. But it had the best mix of nutrients(probably why it didn't mix as fast).
Some people will swear by a brand, and call foul on another.
I also chose sea chem because it was told by a marine biolagist that they used that exclusivly in thier lab.
And the sea chem test kits were as close to those super duper expensive testing equipment in accuracy as you could buy on the open standard market.