I have had 75+ titles in the last 12 years, and there is only one that stands out to me that actually made me a better player- Wheel of Fortune. With the wide flipper arrangement, and the absolute necessity to nudge frequently, and also NOT flipping as much as possible during multiball, the game really changed how I play pinball. After owning it, my scores went up considerably on my other machines.

TSPP, TZ, BSD, LOTR, Shadow, AFM, MM, Tron, Whirlwind and GNR would be my ten.

WoF for exactly what ralphwiggum said, and I'd also suggest adding these:

JD - You must set it up hard, but if it is truly nasty, you really have to think about what you're doing to score high on it.

Count Down - It's a classic Gottlieb game that requires precision in your shooting and handling of one single ball to get higher scores. And often, tournaments aren't just all DMD era titles.

3 Coins - We've used this for a few past MGC tournaments, and I love it because no one knows how to play it. Check out the flipper gap if you think that WoF has much of one. The game requires a real understanding of how important a precision plunge is, as well as a lot of strategy to know when to flip, when to wait, and when to hold your flippers up. It may have short flippers and be old, but it teaches you a *lot* about how to control a ball.

Countdown is a solid choice--may help handle EM configurations too. I would add 2:

WPT--pretty fast game, solid layout with very long basic shots (orbits, ramps, scoops), plus good precision drop target practice and the mini-pf for using small flippers (helps with older EMs). And the level of depth/strategy is second to none, so you can practice strategic assessment too.

Lost World: Fairly straightforward fan layout but extremely fast with balanced scoring. Plus, each of the 5 modes don't share a single shot between them, so you have to be proficient at ALL shots on this game.

And DEFINITELY an EM with long flippers, and one with short ones. (At least one should *not* have inlanes.)

I wouldn't consider TSPP, and only one of AFM/MM to get more variety in. WOF would be very interesting , and most of the shots are also longer (harder) ones.

I think JM is perfect if you want to work on speed, loops, precision shots and close, continuous left and right ramp shots on the left and right flippers (at high speed).

I honestly think that TSPP is a great pin to learn how to stack modes. Yes, the gameplay is long, but it's worth learning how to stack.

No Fear
No pops and a lot of narrow lanes.
No slop shots, they get you an instant drain and a PLAY BETTER!

Gotta have Tommy in that line-up....duh....no brainer lol

Along this line of reasoning, T3 might be a good alternative here. Very narrow lane/ramp shots, and lots of timed combos for them.

Iron Man.

Yup, T3 is another good one even though the "Shoot here und here" gets old rather quickly.
Hurry ups and timed shots always get me jacked up and make me blow the shots so these are essential thing to overcome to improve game play.