How effective are the best regulators?

How bad the ripple is on the output will typically depend on the input-- quality of regulation is often given as a fraction of the input variation. An ordinary kind of regulator would give an output that has a variation a few percent of the input voltage variation. I am not sure what the absolute best ones could do, but googling around I found some that give $\sim$0.02%. (The performance can be temperature dependent.)

There are other criteria in choosing a regulator-- things like the current that can be provided, power consumption, temperature stability, etc. may drive the choice more than output stability.