One that doesn't cause unwanted vibration.
My favorite joint configuration is the Schuler Joint.
If I'm going to play with it, it's not the joint that matters. It's the feel.
Of the lower volume custom makers, I think Tim Scruggs has the best handle on what has to be done to make a low vibration SS joint.
The Schuler Joint is so different, that it shouldn't be compared to other joints. But, it's meant to feel like a flat-face joint. It has more intimate contact than any other joint, except maybe the Layani joint.