For me there are two for vastly different reasons.
Lotte World in Soul, Korea had years ago an indoor coaster in the mall/park (Its a Mall...No its an amusement park!) that was beyond boring with one exception, there was a section that the coaster comes towards a mall walking platform before dipping under...well as far as exciting goes, not much...until the lap bar disengaged!
The Other is Primevil Whirl at Animal Kingdom, WDW Orlando. Its a Tiltawhirl...no its a Coaster...Wait, its both! Either way, gave me bruises and a headache.