> They can't be.

That's not exactly right. What "better" or "worse" here refer to is how much the advantage the attacker with specialized hardware has over you running on a standard server.

This advantage is largely dependent on how much memory the algorithm uses. The more memory, the better. In that regard the order is scrypt > bcrypt > PBKDF2.

More memory is better because this makes running on a graphics card or ASIC harder or nigh impossible.