Bernini's is the one in the group that looks like he means business. The others look like well concieved sculptures. Mercie's is an interesting study of mood, form, and beauty, but somehow doesn't bring me any closer to imagining that it represents the historical David.
In my estimation, all of these works are great sculptures for varius reasons, but none of them created the definitive David whereby you look at it and acknowledge that must be what David was like. For that, we still await the inspired sculptor...