Road Kings is awesome. The rules around spotting the ROAD KINGS letters are well designed, the half-loop is easier to hit than the target banks, but carries a greater risk of a center drain. The Time Lock is tense and challenging, far more difficult than Pinbot's similar Solar Value, and there's a further goal after it in multiball with the Mega Score. Excellent music that I believe was the first instance of a game adding a new melody line to the song as play goes on, and cool sounds, especially the return of the High Speed freeway siren for going up the left orbit. It's also a bit of a sentimental game for me, I played a lot of it in high school.
Big Guns has my favorite theme concept in all of pinball, as well as one of the greatest callouts ("YOU ARE INVINCIBLE!"), but I actually haven't played it enough to form a solid opinion beyond that. It's never been on location where I live, I only get to play it at a yearly show.
I've never played Millionaire. I saw it once, but the displays were out. Pardon the pun, but without any scoring, I didn't see the point in playing.