Greatest ever?
Klein Adroit just before the Trek purchase. So far ahead of its time that it wouldn’t look out of place today.
Integrated press fit bearings, incredible paint, boron wrapped tubes, one piece cockpit, incredible build and ride quality.
Back in the day when it wasn’t XC, it was just mountain biking.
Now to answer your question, a De Kerf would be high up the list, as would a Ritchey. A Cove Ti would be pretty good as lots of people have said, or a Kona Hei Hei.