What is the best bite splint for TMJ clencher?

Depends. There are many varieties of bite splints and no one mouthpiece works for everyone...It depends on symptoms and tooth alignment. There are soft, hard, upper, lower, thin, thick mouthpieces. Need to be evaluated...I would not value a mouthpiece obtained online or at a local sports or pharmacy. Custom made mouthpieces are recommended.
TMJ splint. There are three main types of bruxism splints: full arch hard splints, full arch soft splints, and the nti which goes over the front upper or lower teeth. There are advantages and disadvantages of all three. Which splint is the best to use will be determined by your dentist based on your individual symptoms, including pain symptoms, and whether you grind or clench.
Custom-Made. Obviously, custom-made TMJ splints that have been conformed to your individual bite-print are the best in treating TMJ symptoms, but the trade-off is that they’re much more expensive.
Depends. Depends on what your doc likes. Some like an NTI device. I prefer a upper flat plane splint.