It would depend on whether you mean the most controversial decision or the worst decision (I get the feeling you mean the worst decision).
As far as controversy goes, I'd say it has to be Boyd's non-goal against us in the 0-0 draw. No other decision created the hoo-ha that this one did. Walter Smith and the Weegie Meedja dragged it out to the point where it was still an issue when it was announced that the same linesman would be on duty at the OF game a couple of weeks ago. In reality, it was a non-issue - the whistle had gone for offside, our defence and our 'keeper had stopped accordingly but Boyd stuck the toe in the ball and it happened to go in, a chance shot when the pressure was off - he hadn't tried to score from that distance all night, if play was continuing he'd had closed in before he tried anything (again, as he had done all night). Everyone in the ground knew this but Smith and the meedja blew it out of all proportion with little regard for the facts. This decision caused the most controversy but was not the worst decision in a Motherwell game this season.