Right now there's only really one option for a Beast Master, since Larian has decided to drastically deviate from 5e rules (shocker). You have a choice between 0 CR beasts (Raven with 1 HP, WTF) to a 1 CR über spider beast companion.

5e rules for ranger companion:
It's has a 1/4 CR or lower.
Medium size or smaller.
Add your proficiency bonus to their AC, attack and damage rolls.
Also add your proficiency bonus to skills and saves they're proficient in.
And their HP total is their normal max total, or 4 times your level, whichever is higher.

Larian ignores virtually all these rules with their beast master, ranger companion.