Everyone is always talking about the
otterbox, guess it wins the popularity vote.
However, I've seen quite a few people complain or have issues with them, like the clip failing them or whatnot.
Personally I would the
Ballistic HC case. I bought one for 20 USD on clearance @ mobile city. I think it's out of stock but if you do a google shop search, some vendors will have some. The MSRP is 40 USD I believe, but it's well worth it.
The only problem I have with it is the screen cover gets scratched up a bit (not the Evo screen, just the cover screen), try buying one of those plastic stick ons and it'll be fine. The other 2 minor issues are that it can get a bit dirty so try cleaning about once a week; the other issue is that the cover can be a pain to take off, I pull mine from the bottom opening onto the sides until it's off. Other than that, it's excellent and offers maximum protection like no other.
EDIT: I just saw posters above me recommended Ballistic.
On a side note though, I'd like to add that the case in black gives it this awesome military look. If you theme your phone just right with cool widgets, etc, it looks even more amazing. You just can't go wrong with this case. +the clip never lets go of the phone while at work.