By "best" angle, I assume that you mean "optimum" angle. Optimum meaning the fastest way to get to an upwind mark or downwind mark while racing/free sailing. This is the combination of speed and angle and it will vary somewhat between sailors with different sails, boards, fins, wind and water state.
Maybe someone will jump in here that has tracked his optimum angle with a GPS and can be exact. In the mean time, I will make a guess assuming 0 degrees is dead into the wind:
Upwind = 45-50 degrees
Downwind = 150-160 degrees
This is a guess and may not be correct, since it's based on my recollection of sailing (some racing) formula boards for about 9 years.
Two sailors sailing upwind, one at 45 degrees and one at 50 degrees could arrive at the mark at the same time because the 50 degree sailor will have better board speed to compensate for the poorer angle. I guess my point is that optimum angle will have a range based on the many variables mentioned above.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.