If you're looking for a SINGLE game to improve your overall skill,

*Spoiler Alert*
There isn't one, or we'd all own one.

There's no point picking a game because the rule set is hard, or complicated - as that will only help you IF that particular game is selected AND the settings are the same as what you're used to.

There's no point in picking an EM game with mini flippers - as they don't come up in competition very often and have little chance during play to hone particular skills such as live catches, loop passes, shooting from a cradle, etc. etc.

There's no point in setting whatever machine you do get mega hard through the rules - as that doesn't teach you anything. It just makes it harder to get to features - ultimately making the game become a grind.

There's no point in setting the machine as steep and fast as you can - as that's not the way the majority of machines are set up in competition. It also only helps you learn certain skills, which in some cases are actually easier with a steep machine (catches and cradling for example).

There's no point in setting the tilt mega tight - as you don't learn how to nudge being too worried about tilting. In fact, Keith Elwin has stated time and time again that the best way to learn about nudging is to remove the tilt initially. See how the movements you make to the machine cause the ball to react, see how you can save the ball by nudging, sliding, banging ,whatever without worrying about tilting. Only then introduce the tilt and start tightening it and seeing how much movement you can get away with before causing a warning, or a tilt.

Not being in the position to rotate games constantly, the best ways I have found of improving is by:
a) playing as many different games, at as many different locations, as possible. It gives you exposure to all kinds of standards and set-ups of machines.
b) even if you only have a single game there are still ways of changing the set-up so that it plays completely differently and requires you to improve in different areas.
i)By making the game fast and steep, ball control is less of an issue as opposed to reactions.
ii)By making the game shallower there is more side to side action bringing nudging to save from the outlanes more likely. It also makes it harder to cradle balls.
iii)Adjust the angles of the flippers. You will not always encounter machines set up perfectly so need to be able to adjust to different scenarios.
c)When playing a game, don't just focus on getting the best score. You're trying to improve your skills, not set a GC. In the past I've even covered up the DMD and just focussed on ball skills. SCORE IS IRRELEVANT IN PRACTISE.
d)Try different things - even if they're not worth points or progress the game. SCORE IS IRRELEVANT IN PRACTISE.

e.g. You own, or have access to, AcDc.
Try alternating ramps - helps you improve making shots from a moving ball over and over again.
Shoot into the pop bumpers and see how quickly you can get the ball cradled - helps you improve live catches, dead bounces
Hit the left loop and then loop pass as the ball returns
Practice post passing
Shoot for the TNT drop targets, by bouncing them of the side drops/standups.
Shoot the Thunderstruck drop targets - you'll get a fast return you'll need to deal with. You'll also learn to try and just graze the edge of the target, which generally produces a slower/safer return.
Play completely 'on the fly', no trapping whatsoever - it's surprising how often being able to make a shot instantly is both safer, and more desirable than trying to trap up.
In MB try and trap all balls, and then separate them so you can shoot 1 ball while keeping control of the other.

The list of things you can do to improve on just a single table is massive.

I'm not going to knock, or highlight, anybody above for their opinions, but frankly, some of them are not going to help you improve as quickly you'd think. A lot of the tables mentioned can only really provide limited exposure to the range of skills required.

If you can have more than 1 machine, I agree, that having machines from a range of eras would be preferable, but focus on eras you are likely to meet in tournaments - if that's your main aim.

Finally - whatever machine you choose - it has to be a machine you enjoy playing. You won't be as dedicated in practising if the machine isn't enjoyable to play.