GBA.emu is the best Gameboy Advance emulator in Cydia.
Features that I like include:
-Advanced BIOS emulation. This means you don't need a BIOS file to use it.
-Local directory ROM loading. This means you can load ROMs from any place in your iPad file system. This also means you can load ROMs directly from external storage devices without need to transfer them to a local location on your iPad. If you use an SD card it's almost like plugging in a cartridge and playing a game.
-Save-state support. This includes auto-save by the emulator and saving within the game. You can always start back where you left off. Includes ten slots for save-states so you can save the game at multiple pivotal moments for later use.
-iOS 5.1.1 support. This means it's compatible with your iPad running iOS 5.1.1.
-Native iPad display. This means it utilizes the full screen of the iPad.
-External controller support. This includes the iCade, Wii-mote + Classic Controller, Zeemote, and iControlPad.
-.zip file support. This means you don't have to unzip the ROMs to play them and can load them directly from the .zip files.
-Portrait orientation mode with a 1:1 aspect ratio. This means you can play the ROMs with a large view, yet the controls do not impede the ROMs video display. Can be configured in settings.
GBA.emu is available in the Cydia store for $4.99. It is developed by Robert Broglia, so as I've described it has the same quality you can find in any of his emulators.
So that's my review! I hope that you're able to enjoy this great package like I have been able to.