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 wasn’t good, but it wasn’t all that bad IMO. It was just a game that was rushed in order to use CD-ROMs and FMV. Everyone that had a PC that could play it that I knew owned it though, so I bet that did well sales-wise.
You’re absolutely right that it was nothing compared to X-wing or TIE Fighter (the true GOAT of that sort of game).