Stargate Origins is up there for sure. I'm a huge Stargate fan(which reminds me...I need to finish Ericson on SG: MUSH!) and this was just...not good.
For me, the worst movie I can think of is The Last Jedi. It's not just a bad Star Wars movie it's a bad movie. Beyond that, though it took something precious and dear to me and kind of shat all over it. It did something nothing else had been able to do in 40 years; tarnish my love of Star Wars. There might be movies that are technically worse, have worse acting, or whatever but for sheer emotional impact, this is the one for me.