I'm going with controlled crash. A super Star Destroyer is bigger than the meteor that killed the dinosaurs. But with enough anti-grav working for it, it could control the crash enough to cripple the ship without destroying the planet.
What I find funny, thinking about it now, is how much cheaper ramming an SD into a planet to cause a mass extinction is than building A Death Star... I suppose Dearh Stars don't give much time for escapes though. That makes for scarier symbols.
Forget building SSDs for that purpose. It would be much, much cheaper, easier, and more effective to simply capture a few asteroids/comets in the system, line 'em up towards a planet, and let 'em rip. With all the tractor beams those ships have, this is trivially easy.
