Best boardwipe? Best boardwipe. Why? Let's compare it to other popular boardwipes to see why. Damnation is a good example of the vast majority of boardwipes and their inherent problem: destroy. With predominance of indestructible creatures (especially with Avacyn, Angel of Hope), destroy effects are easily defeated. Well, what about Final Judgement? Exiling all creatures is nice, but for the same mana, you could give everything 4 -1/-1 counters and that will kill a lot of problems or at the very least weaken them to the point of not being problems for much longer. Also, you have to have 6 mana for Final Judgement to work whereas you could have as little as 3 mana for this card to work. Finally, there are the pseudo-boardwipes like Inferno. Now, the 6 damage to players is very nice, I'm not about to dispute that, but again you have to have a lot of mana for this to work (7). And finally, the thing that this has that no other boardwipe has, is that it goes back into your library. What's better than a boardwipe that keeps coming back to kill more?
Pros: Variable mana cost can be adjusted for the appropriate situation, weakens what it doesn't kill