I will point out that there is already a list of these on pinside under bang for your buck:
https://pinside.com/pinball/top-100/bangforbuck
Having said that...this is a bit of a loaded question since the 'fun' level would be different for everyone, and since a $300 pin has a lot more bucks leftover than a $6000 pin, so there are really 2 ways to approach this...but I digress.
EMs are cheaper and were literally made to chew pocket change up, so they are challenging and oftentimes have that one more game appeal. I own 2 that I would put on the list (Teacher's pet...actually rated as the #1 overall bang for your buck) and Wizard. I would throw heat wave and fireball in that mix as well.
If we are looking at SS machines...early Bally or Williams system 11 games have the best bang for buck. Paragon, Flash Gordon, and EBDLE all fit the bill. Highspeed, Pinbot, and Taxi all work well for those slots as well.
DMD machines start to see massive price increases over earlier machines, but there are values like Stargate, world cup soccer, Judge Dredd, and fish tales still out there.
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.