Personally it would be Republic Commando - the books were good too, but I just want a sequel!
This one for me. Such a solid, classic shooter. Only problem was the first level or so was obnoxiously hard, at least for 10 year old me.
For me it was how scary the game got a few levels in. The probe droids with the deserted homes where you get ambushed, oof. Had to have my dad play through those levels while I watched, but he didn’t know where to go so I eventually had to gather the courage to do it myself!