What shots never become routine, always give that sense of satisfaction that you actually hit it. Shots that are hard enough that they cannot be completely mastered, only attempted and made when the moon and stars align and they are struck just right.
I'll give a few I irregularly experience to start off:
TRON Gem shot. Always a thrill to make this oh so tough shot--but a shot that is key to the game and you had better be able to make it--at least sometimes anyway. Honorable mention for the Gem plunge.
TFTC right ramp. So tight, so hard to make--perfect placement or it ain't gonna go in. Even if it does, it more than likely won't make the vertical loop and will drop off into the lower wireform (thank goodness it still gives points for that).
Either Dirty Harry ramp shot. They look so wide open, but they aren't. Tough shots every time.
Defeating "The Power" on TZ. Beating the countdown and getting the magnets to propel the ball up and through the upper playfield hole is always a rush!
Hitting the Super Jackpot on CFTBL. It takes so much work/luck to even be able to attempt this shot, let alone make it. The best shot of them all in my collection.