I have used
IO - nice, have 50 pound bucket now
Reef Crystals - nicer, not worth the extra at the moment as I dose kalk anyway
Kent - my favourite, made in the same factory as the two above, unobtainable in Norway.
Red Sea - all right, not as bad as reputed
Aqua Medic Reef - low in alk i.m.o.
Elos - nice (Italian), but nothing special
Tropic Marin - nice, comes in low in Ca and alk as you're expected to be dosing these anyway.
Personally I like IO/Reef Crystals/Kent, and buy what I can get a decent deal on 25 kilo buckets.