Okay, serious answer:
Time taken for the same person to untie the knot. Repeat across different knots, and different rope diameters, and multiple people. Average results from all different people to get time for a given knot, at a given diameter.
Probably standardise the knot tightening, say 100kg, slow pull, for 5 minutes. You'd also need to decide whether any "tools" are allowed, like carabiner noses or spanners.
To get any meaningful results, you would need a lot of data. I expect you'd find that thin ropes drastically exaggerate the differences between knots.
In 8.5 mm or lower, I've stopped using figure-8 loops at single-bolt rebelays, as they are a bastard to untie. Bowline-on-the bight is a lot easier. Alpine Butterfly is easy, and Fusion is easy.
Maybe the sneaky way to get your data would be to turn it into a new competition at Hidden Earth!