Shock Doctor had good mouthguards, but they changed the fitting material which is too soft now so the mouthguard doesn't stick tight anymore to the teeth and can easily fall out. I have no idea why they have done that.
If you are willing to spend the money ($200-300) I recommend 3 layer custom made mouthguards. It's really a huge difference to those cheap boil&bite mouthguards. My custom made mouthguard is sticks so tight to my teeth that I can't get it out without using my hands.