Treasure is known for producing super-hardcore action games, from frantic run-and-guns like Alien Soldier and Gunstar Heroes to clever shooters like Ikaruga and Radiant Silvergun. Between original releases, they sometimes take on licensed works, seemingly to pay the bills. One of their best games -- and possibly the best game on the GBA -- combines all of these aspects of Treasure's style. It might be slightly strange to say that a licensed game based on a kids' show is one of Treasure's best, but it absolutely is.
Astro Boy: Omega Factor was developed by Treasure in concert with Sega's Hitmaker division (Crazy Taxi). It's based on the Astro Boy (Tetsuwan Atomu) character created by Osamu Tezuka, Japan's analog to Walt Disney (except with interesting characters). The game was specifically created as a tie-in to the 2003 Astro Boy anime which didn't really take off, but it works perfectly well as a standalone Astro Boy work. It follows Astro Boy from his birth through an original adventure based on years of Astro Boy manga, anime, and even other Tezuka works. Astro must prevent a war between humans and robots, and learn about emotions and humanity, as well as the motivations of his mysterious creator, Dr. Tenma, along the way.
While the characters have been rewritten or given new jobs to appear in the game (Don Dracula, rather than being a vampire living in Tokyo, is now Sharaku's henchman 30,000 years in the past), Tezuka himself would have approved. He frequently "cast" characters from one manga as distinct characters in others. This reuse of characters is known as the Osamu Tezuka Star System.
Mechanically, the game is a brawler. Astro has a punch combo, a kick that knocks enemies into each other, and a weak ranged attack (his finger laser). In addition, he has jets that operate as a multidirectional double jump, and three super attacks which are charged up by hitting enemies. Playing Omega Factor involves stringing these abilities together in order to deal with large crowds of generic enemies. For example, in order to deal with an enemy shooting at you from far away, you can kick another enemy into it, or you can jet through the projectile and then attack directly. Most of the time, with so many enemies, you'll be combining such tactics. You may jump in the air and kick enemies away from one side, then punch the enemies on the other side until dead, and laser the original enemy as you fall. The super moves help you get out of tight situations, but are limited (well, limited in name only in easy mode since you can store 99, but you max out at 5 in normal and 3 in hard mode). Other levels play out as horizontal shooters, with an airborne Astro shooting lasers at patterns of enemies.
Astro Boy: Omega Factor seems to be the perfect licensed game. Not only is the gameplay more than strong enough to make the game worthwhile separate from the license, but it makes sense in the context of the license. Even more importantly (for Astro Boy) it tells a great story in the source material's universe and instills a strong desire to seek out more of those stories.