I personally recommend the Shock Doctor Power Ultra. Its $20 and its what I use. I think it is the best mouthpiece I have ever owned. Just this year, I was starting to experience some TMJ disorder symptoms. I replaced my old mouthpiece (Shock Doctor Pro) with the Power Ultra. After wearing this mouthpiece for less than a week, my TMJ Discorder symptoms have all but entirely disappeared!
I will note that this mouthpiece fits a little different than any other mouthpiece I've had before, as it is a much "stiffer" material. It feels almost uncomfortable for the first week or so you wear it, but then you adjust to how it feels and its the best mouthpiece in the entire world!!!! Its also the easiest mouthpiece to breathe with!!!!