Gameboy Emulator (My Boy) and Nintendo DS Emulator (Drastic) are all I need in my android. Let me played a vast range of games. And My Boy even supports multiplayer, e.g. you can battle /trade Pokemon, duel/trade Yugioh, fight Tekken, go to war in Advanced Wars, battle Megaman Navi/trade Battlechips with another player.
The best part is no internet connection is required. Though it is not Massive Multiplayer Online so you can only play with your friends when you are physically close to each other.