So far the best I've personally tried is the Solarbotics Pi S.A.F.E. V3 .
But it's still not perfect, with rather small gaps for fan airflow, and the included screws are a very tight fit. But I like it well enough, and it looks good sitting next to my TV and the like.
Still, I wonder if there's not a better choice that is both reasonably priced, and fully encloses the Pi.
BTW - I need active cooling because I run BOINC 25/7 when I'm not otherwise using my Pis. I hate to waste CPU time :-)
And I need to use a 40mm+ fan, because I've yet to find a quiet smaller fan. I wish I could find a quiet 30mm fan, as cases with 30mm fan mountings seem to be incredibly common, vs larger mounting sizes.