I wasn't going to mention trazodone because it also doesn't work consistently and is notorious for that quality. It mostly helps me, but it is not the most sedative of meds I have taken.

The most sedating medication I have taken is mirtazapine. It is an extremely potent antihistimine, so it knocks people out. At this time mirtazapine will knock me into the next week, but when I first used it my insomnia was so bad due to anxiety that I still slept 3-5 hours a night. I was also on effexor at the point, and it may have been over activating. An activating med could be counter acting any sedative. However, I always hear seroquel being talked about as though it is a much more powerful sleeping aid.