To keep things a bit simpler, I'm going to limit my options to games I actually own and/or have played. In alphabetical order...
Dungeons & Delvers (both Black Book and the kids' version): you get a handful of various characters confronting a dragon (but it could have also been another monster or even group of monsters).
Black Book was intended to be a kind of homage to the "black box" Easy to Master D&D Game, and while I considered having some guy confronting a dragon, I didn't think that captured at all the feeling of the game. I actually did the kids' game cover first, but at most it subconsciously influenced the d20 game: I just really like the look of red dragons.
For a similar reason I like FantasyCraft's cover:
An actual group of varied adventurers slaughtering a horde of varied monsters, with a heap of treasure before them. The thing is drawn as if you're looking out of an even bigger monster's mouth: maybe it's from the perspective of some poor bastard just about to get swallowed up?
Shadowrun (2nd, 3rd, and 5th Edition) also does a pretty good job: you get a group of runners dealing with some shit. I really liked the 2nd Edition cover (which was also the first edition that I played):
While I'll never play 5th Edition (not for a lack of trying) I think it's cover is pretty fucking rad:
I think it's better than the 2nd Edition version, because a bit more setting stuff is conveyed (namely more overt magic, the corporate buildings and drones, and you get to see an orc, dwarf, and troll). I also think it just looks better overall.
And now for the bad, starting with Dungeons & Dragons. While 4th Edition shows all of two characters, usually you end up seeing just one character confronting a big monster, such as a dragon...
...or another dragon.
Rifts was also bad because it shows a weird tentacle monster surrounded by sexy women and these floating eyeball-glowing-skull things.
Rifts takes place on Earth in the future after a punch of portals to other dimensions vomit a bunch of aliens and monsters onto the planet, and magic, psionics, cybernetics, and robots are also a thing. It's been awhile, but I'm pretty sure that the above thing (a splugorth slaver with blind alterian warrior women) isn't even described in the first book, but the Atlantis (and maybe in other books).
After putting it to a vote, the next couple of classes on the docket are the warden (think 4E D&D warden) and apothecary (gotta go see what they're all about).
Dwarven Vault is our sixth 10+ Treasures volume. If you're interested in thirty dwarven magic items (including an eye that lets you shoot lasers) and nearly a dozen new bits of dungeon gear, check it out!
Just released our second adventure for A Sundered World, The Golden Spiral. If a snail-themed dungeon crawl is your oddly-specific thing, check it out!
By fan demand, we've mashed all of our 10+ Treasure volumes into one big magic item book, making it cheaper and more convenient to buy in print (which you can now do).