There are so many, but I always liked this image. Not so much because the art is good (which it is), but because it conveys the sense of adventure. The protagonists aren't sleeping in comfy beds, they're stuck gods know where during what seems, at least, to be a calm and starry night, taking comfort in their campfire and each other's company. There is no conflict yet and they don't look like they're expecting trouble immediately, but they're obviously up to something.
...and it looks like we just walked in and disturbed their fireside lute-playing, waiting for the "please join in, stranger" or the "get the f**k out before you upset our mage here..,"
I'm always a huge fan of Willingham, probably because he reminds me of Mike Ploog's style (which I love). Outside of this one, I always default to these classics:
I'm not sure I could name a single favorite, but this Clyde Caldwell piece (from the AD&D 2nd Edition DMG, I believe) has stuck with me for quite some time: