What CSU offers the best biology master degree?
Best is pretty subjective.
Without knowing what you plan to do with it -I’d suggest you start with SLO, SDSU Long Beach and CP Pomona.
Most CSUs have strong reputations in their own backyard. So, if you wanted to live in the San Joaquin Valley, Fresno or Sac would be a fine choice.
What do you hope to accomplish with your MS?