It seems that the biggest crater in the Solar System is Borealis Basin on Mars. The Wikipedia entry and this piece of news say it has been formed by an impact with a body of around 1600 - 2000 km. Mars has a diameter of ~7000 km. The impact velocity was estimated as low 6-10 km/sec. Mars has its own momentum and speed, and I assume collisions in space are plastic.
Instinctively, I have some doubts about these calculations, and I am curious how this estimates are done. I am quite sure that these guys know a lot of physics, but what I see is one big ball colliding with a ball a third of its size and leaving only a scratch? Can you land something up to 2700 km big and only get a basin? Was the other body a big pillow? Maybe it was a touch and go? How come Mars was not pulverized to bits?