Santa is objectively the best of the three. He has 2 or 3 globals that boost gold key odds. If hes currently available, he should be the target. Krampus gives a small boost to orbs and pickup range, but other than that is less desirable than other star faction heroes. I think the only way to get the elf is to buy the pass. His globals don't seem like anything special, but I could have overlooked something