This is a little tougher because I don't usually distinguish between cover art and interior art - I tend to think a game looks good or it doesn't.
That said, my initial thought was the FFG Star Wars line because it is really well done, consistent across books and lines, and makes for some very pretty game books. I don't know that it's really "inspiring" though. The Star Wars movies and shows and books and comic books and video games are already pretty inspiring and so it's more reinforcing an existing thing than starting a fire on its own. Also, while the illustrations are both numerous and well done they do not generally show you things that your characters might be doing during a game - they're Star Wars pictures for sure, but they are not really Star Wars RPG-specific pictures:
- Here's a star destroyer flying near a planet
- Here's Han Solo sitting at a bar
- Here's a droid peering through some binoculars
- Here's a scout walker
- Here's a couple of characters standing still and looking "at the camera".
So while they look good they are not really "inspiring" me to get a group together and roll some dice. They confirm the setting but do not really enhance it, in an RPG sense.
So now that I've talked about what doesn't do it for me, here's one that does: Dungeon Crawl Classics. Why?
- It's all strong black and white art.
- It tends towards a "weird" vibe. You don't always know what you're looking at.
- It shows things that could easily (and maybe should, easily) happen during the game.
It does, for me anyway, build an interest in playing the game as I read through the rulebook or an adventure. Heck, even the maps are more evocative of some lost document than in most other games and yet they remain as usable if not moreso than most others.
Yes, some of it is the old-school thing which does have a certain appeal to those of us who started in that earlier era.
But the very unexplored/unexplainable/rough around the edges tone of much of the art enhances the atmosphere of the game. We don't know everything about the setting. We can;t instantly identify every monster that is shown. We don't know why that character looks that way and this other character looks completely different as far as dress, gear, and attitude, and in the game you aren't going to know everything either!
When it comes to art-matching-expected/designed-tone-of-game I can't think of a better example than this game.
One more for the road: