All right, I have two suggestions.
First of all, there's Gameboid. It's an excellent emulator, and it should work with just about every game you throw at it - however, a lot of them may be buggy and the sound is just downright awful on many of them.
Next, there's GBA.EMU. You can download that from the Play Store for, like, four dollars - it's updated at least once or twice a month, support is growing, and it seems to me to be the more powerful of the two emulators I've listed - however, it's missing some features, such as a real-time clock (I tried playing Boktai on it, and it was perpetually 12:00 AM, as well as Pokemon: Liquid Crystal). However, this should get better as the emulator matures. The developer is mature, professional, and seems to know what he's doing.