That is a very contriversal question. Let's start of easy. I like mixing my own water to maintain consistency. People would come into B+B all the time complaining that their salinity was never the same. Found out they were getting premixed water from places like Exotic Aquatics. After bringing it to their att., they would test the water when they got home. After a couple times they informed me that it was different every time. Consistency is important, especially in a reef. Now on to the hard part. Everybody will tell you to get the salt that they use cause that's what's working for them. No salt is perfect. I've tried: Coralife, Tropic Marin, Red Sea, Instant Ocean, Reef Crystal's, Oceanic and now I'm trying Marine Enviroment. RC was ok but not enough cal. Oceanic seemed to be working fine for me, regardless of what other people have said about it. Matt from Shotanks was telling me about Marine Enviroment. It is suppossed to be a great salt for reef. I just started using it, so I don't have an opinion on it, yet. It seemslike a good salt and I can see myself sticking with it. The company that makes it also has other versions for less than reef situations. Which ever one you choose, keep on testing the water to keep track what that salt is doing for you. Example: Oceanic has worked well for me. I have a friend that tried it and he said his water was out of whack with Oceanic. So try one and don't be affraid to pull out the test kits.
I have used many and am happiest with Tropic Marin.
IME it is the most consistant and mixes up closest to NSW compared to the others I have used. Its a bit more expensive, but when the cost is spread out over the usage time of a bucket, its not all that much. The water is the most important thing in our tanks... Why skimp