I use regular IO.
Here's my standard commentary on salts. Some of it may be a bit dated as I've not kept up on every formulation change to every brand:
I do not think there is a "best" salt mix. Nearly all of them will work fine as long as you know their pros and cons.
I don't want excessive borate, which leaves out Seachem.
I don't want vitamins or anything else organic in my mix (because I doubt their utility, they degrade with time to who knows what, bacteria may thrive on them as I store new salt water for a substantial period, they are totally undescribed with respect to amounts or identity, they are often not naturally present in natural seawater at appreciable levels, and because I've occasionally had them mess with my skimmer), so that tosses out some like Reef Crystals, hW Marinemix Plus BioElements, Kent, Coralife, and Nutri-SeaWater.
I don't want excessive calcium (long term use of limewater as I use drives up calcium, so I do not want it starting high), so that tosses out a bunch, such as Kent, Seachem, Coralife and Oceanic.
There are certain companies that I will not support due to their misleading claims and/or product lines. That tosses out a few which I won't detail here since it is my personal thought as opposed to a specific issue with their salt mix.
I won't use certain lines of natural seawater due to excessive metals in it.
That only leaves a few to choose from, such as Instant Ocean and Tropic Marin Pro. The remaining ones might all be fine for me, but IO is lower in cost, especially if you get it when it goes on sale (which it frequently does). It also has a very long track record of success in many aquaria with relatively few concerning issues of bad batches.
