For the online competitor...Metroid Prime Hunters (read our review)
Nintendo's online presence still hasn't reached the near-perfection of Xbox Live, but the Wi-Fi Connection is definitely a major draw for the hardcore audience. No one really thought a full-fledged FPS could work on a portable system, but Hunters pulls it off. Deathmatch, capture the flag, defend the base... any kind of versus match you can engage in on a console is done here. And with seven shape-shifting bounty hunters to choose from, each player can approach the fragfest from a different perspective.
Take it offline and you've still got a robust single-player game that rivals the critically acclaimed Prime console series. Whether it's multiplayer or solo, Hunters remains the definitive DS way to shoot first, ask questions later and still throw in some light puzzle elements to give your brain a workout.
Second choice: Star Fox Command