I'm looking for a good telephoto lens to rent for my Nikon d3000?

I'm going on a safari in a few weeks and I'm looking to rent a telephoto lens. I want something with very sharp images, but also a lot of range. I've been looking at the Sigma 50-500 but I'm not sure how sharp that will be, and I also want a larger maximum aperture.
Any ideas?

Wide aperture telephotos and zooms are very expensive - and not cheap to rent either!

If you can afford it get the Nikon 200-400mm F4, but that's going to cost a lot.

I would consider the Sigma 120-300 f2.8 and a 1.4x converter. With the converter you would only lose about 1 stop of light, which would effectively give you 180-450mm f4.0, which is still pretty quick. If you took a 2.0x converter as well you would have the option of a 240-600mm f5.6 equivalent lens if you really needed the extra length, but without the additional weight.

That should still be reasonably sharp and a lot cheaper to rent than a Nikon 200-400mm f4 VR.