lol,
My worst buy would be my first! It was a Super Mario Bros... It had 1 incorrect leg, It was missing half the ball eject assembly, backglass nearly smashed when i lifted the backbox up because it was missing locks and the skirtings, all the playfield flashers were blown, 80% of globes were blown, paint flaking on upper playfield, cabinet was scratched, both flippers didn't work and had incorrect assemblys, and there was a problem with the roms... most of the gameplay was missing.
Anyway...
My best purchase was a local one.. A Dr Who.
Not just am I a fan of the series and movies of Dr Who but I think the game has a challenging gameplay with its 7 features and artwork, I bought it in mint condition absolutely nothing wrong with it for $2700 inc delivery with a slight fade on all sides of cabinet. I believe the machine is currently worth $4500