Having played the vast majority of games with a release date from 1989 onwards, as well many more pre that, the least enjoyable have definitely been:
Harley Davidson (Bally)
Rocky & Bullwinkle (Data East)
BB Birthday Ball (Bally)
Of the games I've owned
Hook (Data East)
Aerobatics (Zaccaria)
And the winner is Hook - a game with absolutely no redeeming features!