At first, I thought I was going to have a difficult time with this question. I didn't know how I was going to pick just one illustration as my favorite, but one obvious contender made the decision pretty easy. This piece is one of the most iconic images within the hobby, having graced the cover of one of the most well-remembered starter sets ever released, the classic "Red Box".
Due to being born in 1992, I never had to chance to experience the original Red Box, but I'm very familiar with the image printed upon that cardboard lid. I don't know about you, but this Larry Elmore piece just screams "fantasy adventure" to me. You have a badass warrior who looks like something out of a Robert E. Howard story, fighting a ferocious dragon atop a mountain of gold within an ancient ruin. Just add a wizard, a rogue, a dwarf, and an elf in the background and this image would perfectly capture the iconic Dungeons & Dragons adventure. This image just makes me want to grab a sword, find a dungeon, kill a dragon, and loot its horde of treasure.