This is my favorite boxart of all time, and *gasp* it's from a PC game (just ignore that ugly sticker in the lower-right hand corner):
Also, for a joke boxart, a really good one is the one that illustrates this quote about Halo 2: "Halo 2 is a lot like Halo 1, only it's Halo 1 on fire, going 130 miles per hour through a hospital zone, being chased by helicopters and ninjas, and the ninjas are all on fire, too." I couldn't find a pic of that boxart but it's been posted before so hopefully someone can dig it up and post it.