Gameloft's Dungeon Hunter is probably the best Diablo-esque action RPG on the App Store right now -- it looks great, and plays just about as well as any touchscreen controlled action game can. As a Warrior, Mage, or Rogue, you can hack and slash your way through a fantasy world, picking up armor, earning XP and exploring a serviceable (if not particularly gripping) story. The save system is a little annoying, and the inventory can be a bit overwhelming, but the presentation does justice to the genre on Apple's small screen.
In accordance with an ongoing sale, Gameloft has the title priced at just 99 cents. For cheap action gaming thrills on the iPhone, things don't get much better than that. There's no word on exactly how long the game will be so cheap, but if the idea of a portable Diablo clone appeals to you, open this treasure chest right away.