Bleeding brakes. What brake fuild is best to use?

Whatever you are going to use as the 'service brake' fluid!.... DOT-3 normally.... DOT-4 if 'towing heavy' down a mountain using truck brakes often! ... BTW... It's worded right on the top or cap for the master cylinder!!!! Dot 4 simply increases boiling point....

Valvoline synthetic is generally a good balance of price/performance for Dot 3/4 applications.