Aquarium Systems claims to have made IO (Instant Ocean) easier to dissolve. I haven't really noticed any difference between the old and the new. I've been using IO almost exclusively for the past year and it's always dissolved fairly easily for me.
I've also used Kent salt and I never had any problems with it either. It's just slightly more expensive than IO in my area.
Some people swear by Reef Crystals, also made by Aquarium Systems. I haven't tried it yet, but I hear it's one of the more expensive salts available.