Spike traps on my favorite. They get me the most often, but they're the most entertaining in design. There's one, I believe in Shriekwind Bastion, where one of those stone pillars raises the victim into a hole/pit in the ceiling and the spikes shoot out from the walls across the whole surface of the pillar. Pretty grim.
Swinging spike walls are the ones I'm most often able to use to my advantage, and they tend to do the most damage after a dropping lantern.
My least favorite are trap doors. They're easy to forget about, although they're also usually pretty easy to spot.
Falling boulders. Physics allow you to kill any enemy with those.
Swinging axes were very helpful during my no-kills run. Lured the draugr there
Spear traps in a locked area: Poor Wyndelius
Skull ram because of TUSKS!