Max Payne 2
Deus Ex
Ico (the original art)
Diablo
These images are all striking, elegant, and iconic. They capture their games perfectly. Elaborate brush paintings and space-opera composites are nice and all, but the ones that stick in your mind are the simple, evocative ones.

Exactly. There was no need to post sample pics because everyone here remembers exactly what they look like. There's also a certain bold confidence about the game in the minimalism of the cover. As long as that confidence is well placed then it's something that pays off superbly.

Seconded (or thirded, as Mentalepsy mentioned it too). Ironically, ICO also has, in my opinion, some of the absolute worst cover art ever. Witness the butchering.
Honestly, what the HELL were they thinking with the redesign?
Actually, I think I know: American covers for anything have strict rules about the prominence of the protagonist's image. They have to 'tell a story', which is absolute crap, because 90% of the time it results in same-y garbage that just looks like (and likely is) a poor photoshop collage of stills and concept art from the game/movie/TV show.
I also find it interesting to note how many covers are just stock religious imagery with the lead character subbed in for Jesus.

That really is awful. Who'd look at that and want to buy the game?

As many of already said ICO (it's just a fantastic cover, you can't mistake any game for it), Another World has a really awesome cover and the Prinny box art is pretty great too (http://upload.wikimedia.org/wikipedia/en/1/14/Prinnygamebox.jpg)

Well, you really CAN'T beat Monkey Island box art....
http://emigrationfordummies.files.wordpress.com/2009/09/the-secret-of-monkey-island_0big.jpg

While browsing through my collection of games, I recalled this thread, and I have two games to end this thread:

One of the editions of Falcon 4.0 comes in a freaking technical manual. I can't find it right now and I'm not about to go ransacking my cupboard at 2 in the morning to find it, but believe you me, it's the only game I know of which doesn't only ship with the manual, but also ships in the manual.

And then there's Simcity 1. Not the improved Deluxe edition, but the very first one produced in 1989. Seriously, if this doesn't end the thread, I don't know what else does =D

-edit- Oh yeah, there's one more classic that I read about on RPGCodex, and I've since gone on a scavenger hunt for it on eBay. I spotted a copy of it at a local store almost a decade ago, but I didn't pay any attention. It's been taken off the shelves now. It's too bad no publisher has since reproduced a box that takes the shape of a monster's face. (You read correctly: the box is shaped like the face, not printed like it)

The Horde