Well, "best" is subjective, most accurate out of the box, there are several that come to mind. In no particular order, here is a list of muzzys that either have a reputation for being very accurate out of the box or ones that I have shot or owned and can confirm that they are indeed accurate out of the box:

-TC Triumph
-Knight Vision
-Knight Disc Extreme or Elite
-CVA Accura
-CVA Optima

There are more that I'm forgetting, I'm sure, but this should at least get you started looking. And if I'm not mistaken, you should be able to find all of these NIB in the $200 - $550 price range.

I may get some hate mail for this, but the only muzzy that I would NOT recommend if you are looking for accurate out of the box is the TC Encore. Don't get me wrong, the Encore is a nice rifle, it just has the reputation for needing some tweaking to get the best accuracy out of it. That and it is overpriced IMO.