Quoted from Daditude:

Now let's cut to the NITTY GRITTY: If you've got 6k and you want the absolute BEST possible game for that price...here are the true gems:
Star Trek The Next Generation - this game can be had for 6k and it's LOADED. Mechs, art, call outs, theme integration, rules, gameplay, scoring, and flow are all top notch. It is always consistently considered one of the top pinballs ever made by pinside, ipdb, and every other worthwhile list. By today's pinball prices, it should cost 12k.
The Shadow - Many consider this to be one of the most underrated games of All-time...I'm one of them. Serious pinheads that have amazing collections swear by this game. The layout is truly unique, so it would fit in any collection. The gameplay is masterful. It is an amazing pin.
Honorable Mention
The Getaway: High Speed II - this seems to be forgotten by many on these lists, but at 5k and under it needs to be brought up more. Game flow, rules, one more play appeal, art, and mechs/gimmicks are all great. I feel like there is no one thing you can say is the 'best' in pinball, but the sum of its parts are fantastic...which is probably the only reason it isn't several thousands more.