A playable work in progress, Pixel Dungeon is already one of the best Android has to offer.
Pixel Dungeon is a rougelike dungeon crawler. You pick a class and descend in killing everything that moves on your way down swinging swords, casting spells, drinking potions, and tripping traps. The usual stuff, but updated perfectly for touch play. You’ll probably die a couple hundred times because you’ll want to keep playing even after you’ve already died a hundred times. But you’ll get a little bit further and a little bit smarter most every play through.
One quick caution; the scrolls runes and potion colors are random each play so don’t bother trying to remember them. If you want to jump ahead of the learning curve check out the fan service wiki here.
Pixel Dungeon is generously free, available at Google Play, but when you find yourself playing it for hours I encourage you to donate at the official website to promote updates.