A game that shows off stellar graphics that really pop off the screen is Jet Raiders, a flying adventure through detailed and beautifully crafted levels.
iPhone and iPad apps tagged "top-down shooter."
Team Alpha: All the Mutant-Shooting Action You Can Handle
Team Alpha is a shoot-em-up, action game where, according to Sad Cat Software, your task "is to eradicate all mutants in a government lab where a secret biological experiment has gone awry." The app offers nearly 200 rooms of mutant-shooting mania with big guns.