Pick one from Genie, Big Shot, Fish Tales, and BSD (others may, and almost certainly will, disagree).
BK2K doesn't really get in there because it's easy to keep the ball in the upper playfield and spam loops.
I'd agree with these too although for me I would swap out ST:NG for BSD. I still can't get a handle on playing ST:NG well after all this time. I'd also add Firepower due to the table speed. I find it so easy to lose control of the ball(s) and can't regain control before they're shooting down the outlanes. I'd also add Ripley's. The table play is not bad, particularly when you identify all the dangerous shots. But the complex rule set and difficulty completing all the goals is challenging.