what does best brainrot in steal a brainrot actually do?

lock your base. there's a lock button in it that keeps people out for a short while, and early on that's basically your only defence. your best brainrot does nothing for you once someone's carried it out the door while you were at the shop.

it generates money per second, that's the whole job. the higher the rarity the bigger the per-second number, and that's all "best" really means in that game. the catch is the good ones are exactly what other people turn up to your base to steal, so a big earner you can't defend is worth less than a mid one you actually keep.