I look for justices who are strong on the Bill of Rights from start to finish. That includes taking seriously its dictate that rights are expansive (9th) and powers are limited (10th). Unfortunately, it's very common for many justices to fall into the trap of picking and choosing when to be strong and when to be excessively deferential in a pattern that (surprise!) matches the legislative goals of their respective political parties. And too often, "moderate" judges will almost exclusively break with their party in ways to become weaker on the Bill of Rights.
That's why I'm happiest with what I've seen from Gorsuch. He's not only principled in a way that I expect a Republican appointee to be, but when he breaks from party lines, it's usually in ways that Republicans are bad.
I struggle to see a counterpart among Democratic appointees, but I will say that Sotomayor still manages to be my favorite, because she's often very good in the ways that Democrats are good.
I mean Sotomayor literally wants the judiciary to repeal California proposition 209 because it violates equality or something.