Pretty much all of them. I still watch just to see the horses, but nine and a half out of ten times the movies suck.
One in particular that was just god awful was "Wildfire: Arabian Heart" or some crap like that. That had to have been the worst everything. Story/acting/production. BLECH! Even the adorable arab in the movie wasn't enough to make me keep watching.
On the other hand, I did like Misty of Chincoteague, Into the West and King of the Wind. No others spring to mind off hand.