In terms of the Wing Commander games, my favourite is probably a toss-up between the original Wing Commander PC or FM Towns box art.

I enjoy that the PC box art captures how the game might look when you recall it in your mind - perhaps more literally than any other cover I can think of.
The FM Towns cover is just top-tier. Yeah the characters aren't recognisable with 100% certainty IMO, but they're still more or less on-model otherwise (hilariously-oversized gun notwithstanding

).
In terms of game art in general? That's incredibly tough.
It's kind of the opposite of the Wing Commander ones, in that it's pretty abstracted, but the Shadow of the Beast cover has to be one of my all-time faves (it's by Roger Dean, one of my favourite artists, so that helps):
I also absolutely adore the art on the cover of Ico:
Honestly I could probably post a thousand things, there's just... so much.