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!
Star Wars: Rebellion by a long shot. And that's saying something considering how much I loved Jedi Academy and X-Wing vs TIE.
Having characters run missions that shaped the course of the Galaxy really helped the game write its own stories in the Star Wars universe. Things could happen like Chewie becoming force sensitive, getting trained by Luke, and then Chewie could lead a mission to blow up a Death Star!
The constant cat and mouse game between the Rebels and the Empire was exciting. The game has kind of been reimplemented in a board game by the same name by Fantasy Flight Games. The board game is good, but there's still magic about the original that I haven't seen replicated anywhere else.
Rebellion was one of my first strategy games and I absolutely loved following the careers of all the characters I read about in the books. It's what built the foundation of my love for grand strategy games like Stellaris.
I’ve always loved Galactic Battlegrounds because RTS are fun. Rebellion is another great one that another commenter mentioned. I was also a fan of Jedi Knight and Jedi Academy and replaced them on switch a year or two back.