For me, it has to be Alien: Colonial Marines as it’s terrible due to inconsistent frame rate (moments the game ran smooth and times where lag was insane, even with the best hardware). Both player & enemy AI is crap since the combat wasn’t even that immersive plus Xenomorph AI isn’t as intimidating due to it being poorly implemented.
Rebel Assault was neat, but the controls were terrible. Using a joystick to point a cursor on the screen is frustratingly inaccurate - and not all joysticks were created equal. Cheaper ones were inaccurate and had weird axis profiles thta made the game even harder than it should have been - and trying to play it with mouse was at least as frustrating.