Magic resistance, i.e. advantage on all saving throws vs. spells and magical effects.
By itself, it possibly isn't too powerful, but yuan-ti pureblood paladins of the ancients are, apparently, very popular, and with their aura benefits... the synergy is extremely good. We've had one in the party since the beginning, and it is that good
It depends how many enemies are casting spells at you (and consider that enemies might identify your race, know you are magic resistant, and try something else).