Curling is actually a mentally challenging sport, moreso than a physical one(obviously). To appreciate it, you have to be interested in outthinking your opponent rather than outhitting/outskating/outthrowing, etc. It's a fun sport to play, decent to watch. But it's far from the most boring in either category.
Golf is definately one of the most boring to watch, but I find it one of the most fun and challenging to play.
I can't really think of the most boring sport to me, although that equestrian dressage sounds pretty bad.