That's just more time spent with the admins having to read every post. And then who's to say what the admins deem the best post? Some people hate my posts, and others love my posts. 'Best' post is very subjective. I see posts i think are aggravating, yet the OP gets something out of it and likes it.
Between all the time it would take, and the subjective nature of this concept, i don't see it being practical.
Also, what's deemed an 'old' post? What if someone brings up an 'old' post and adds to it? And that post is 'better' than the 'best' post?