Even when I was younger, I was pretty good at not wasting my money on absolute shit, so I haven’t played a lot of the truly horrible stuff. Of course, however, some garbage inevitably crept in.
Probably the worst game I ever played...
E.T. on Atari 2600. My parents and I had this for our Atari and there was just so little fun to be had. Very slow, very tedious, and the controls to float out of pits were awful.
Dishonorable mentions:
AD&D Heroes of the Lance (NES) - I didn’t actually buy this one, it was a birthday gift. Horrible game design and hit detection.
Winter Games (NES) - Awful graphics and sound, somewhat playable but not a lot to it. Only 4 events available.
Fist of the North Star (Game Boy) - A one-on-one fighting game like Street Fighter, but with comically high jumps, maybe 3 moves per character, and (IIRC) no blocking. I sold this game because I couldn’t stand it.