Here's a list of some of the better games that I've found...
Dungeon Defenders - The First Wave -- A mix between tower defence and Diablo II type dungeon crawler with lots of loot and leveling up. High system requirements though. but I managed to get it running on my Droid.
Pocket Legends -- World of Warcraft type MMORPG
Zeonia and Zenonia 2 -- Two very solid JRPGs... Graphics are 2d and very similar to Legend of Zelda - A Link to the Past
Angry Birds and Angry Birds Seasons -- Like tp76 recommended. Fun, addicting and great for playing in small sessions.
Need for Speed Shift -- Racing game.
The SIMs 3 -- It's the SIMs on your phone!
Alchemy -- There are two of these out by different developers...both are good. There might be a new one out as well.
Spectral Souls -- Fun Tactical RPG. Very pricey though (like $13)
RoboDefense --
This is just a few of the games I enjoy, and with provide you with plenty of game time. Most of these cost $ though, but there are.free versions of most of them. With the exception of the Angry Birds games as.mentioned above. They aren't free on the iPhone thoughx so consider then a gift