I can tell you what "great" means to me, in 2(.5) very clear words.
"You (fucking) Ship."
To me a great dev has a track record of delivering the end result. A greater dev delivers the result faster, with less debt attached.
One can fairly say that this is still subjective, but my criteria are _quite_ meaningful and even somewhat empirical/measurable, although it's admittedly hard to do in a short interview format. I certainly know a "Great dev" when I've met/worked with them, because they ship faster than I do, with less friction or need to revisit post-factum.
Edit: Since half the responses seem to elide a very key part of my statement, let me echo: _debt matters_. Having someone who ships but burns your team down, not good. Someone who ships but leaves you with years of bugs or un-maintainable code, not good. As I'm sure we all know, you get optimizations against KPIS you measure, and if you don't measure "delivery" holistically, you'll absolutely get people who excel at one component while salting the fields.