The worst game I ever bought has to be The Great Space Race on the Spectrum. What a turd of a game, I really liked Valhalla, the previous game from the same developers so I mistakenly thought this one must be great too, after all it cost £14.99 (which was an awful lot of money for a Spectrum game) and came in a box the size of a small country. It was so appalling that I corrupted the tape so I could return it to try and get a refund, I sent my mum to take it back and she came home with a replacement copy
I still have it boxed up in the attic somewhere.
The Great Space Race (ZX/C64, 1984)
I'd also like to nominate Deus ex - Invisible war just for the sheer dissapointment value and Halo on the PC, it started out great but those later copy and paste levels where you had to follow the camp flying robot thing while shooting endless waves of blobby balloon people was brain liquifyingly boring.