Biggest crater in the Solar System 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?
 A: It didn't leave "only a scratch"; it appears to have melted about 40% of the surface of the planet, creating a basin that's still visible at least 3.9 billion years later.
It wasn't big enough to shatter the planet.  Assuming it was 2000 kilometers in diameter, that's only about 2.5% of the volume of the planet.
Shortly after the impact, the effects would have been much more visible.  It's likely the other 60% of the planet's surface was strongly affected as well.  The surface has been extensively modified since then by volcanism and other impacts.
Leaving 40% of the surface of the planet "one of the smoothest surfaces found in the solar system" is about as far from "only a scratch" as you could possibly get without destroying the planet.  Mars is big enough that after any impact that doesn't completely disrupt it, it's going to return to a very nearly spherical shape.
EDIT : And since the mass of the impactor was presumably incorporated into Mars, it would have made Mars itself a bit larger; if Mars is now 7000 km in diameter and the impactor was 2000 km in diameter, then Mars before the impact would have been about 6945 km in diameter.  The surface of Mars is now effectively about 27 or 28 kilometers higher than it was before.  (Or some of the mass may have been blown off into space; it depends on the nature of the impact.)  The surface you see now is largely the result of that impact.  And if you had seen it shortly after the impact, the appearance would have been dramatic.
If the impactor had hit at a shallower angle, it's likely that Mars would have ended up with a sizable moon.
