Id recommend SiN. I personally slammed out SiN: Wages of sin, the level pack but the original plays well too. Xibalba is another nice game if you like doom-esque gameplay. Turning point fall of liberty is a tough one but really rewarding at times. The toughest one ive tried is Realms of the Haunting, just a few you might like, all relatively low-key. (Not sure about how many of these are gamecube or GBA whoops!)