Masters, but I always bring my own. It's night and day difference between what I get at the pool hall. My theory?
My own chalk sits in my climate controlled workshop, 70 degrees, 35% to 45% humidity. When it comes in, I leave the box out for a couple of weeks, and then I seal it in a Ziploc bag. When it sits in my case, it sits in a plastic ziplock bag. My chalk is DRY.
Pool hall chalk? It sits out, and all the air conditioning/heating/whatever gets turned off when they close for the night. In most parts of the country, that means it's going to be sitting in a high humidity environment for a significant portion of the time, and it's going to get a little humid.
My Masters is bright blue, clean and goes on beautifully. Their masters is dull looking and applies kind of randomly.
Again, just my opinion, but I've been carrying my own chalk for over 20 years and it's always been the same, flag or no flag. If you don't like Masters, at least be sure to order some fresh chalk and bring your own before dismissing it.
If I ever see someone with a cube of Magic chalk, I'll give it a try.