I too have a very hard time picking out a favorite on the Cube. I would be torn between, Zelda: Wind Walker, Metroid Prime, Viewtiful Joe, P.N.03, Skies of Arcadia, Ikaruga, Shadow the Hedgehog, Starfox: Assault, and Tales of Symphonia. There are so many that I love playing, it is hard to narrow it down to one.
I love the game. It is a great shooter. I feel it is very under rated. Once you get the hang of the controls, it seems to be natural dancing around the enemy fire and blowing them up. Plus I like the different suits with the different power ups.
But, some things might be a matter of taste. That might be the case of P.N.03. When I look at all the reviews of the game, it looks like people either love it or hate it.
edit: You really have to be a shooter fan to like sometthing like P.N.03.