Whats your price range?
The 70-300 f/4.5 is around $400-$500
The 80-200 f/2.8 is around $900
The 70-200 f/2.8 VR is about $1,500
The 70-300 G lens can be had for $100, and it is a pretty darned good lens for the money.
I am a fan of the Tamron 70-300 LD DI Macro, which is light and cheap and very sharp.
The 80-400 VR is a good lens for longer shots, but it is kind of slow and kind of pricey at $1700.
I have the Sigma "Bigma" 50-500, which is a BEAST of a lens. It is fairly sharp considering the range of the lens... and I will make ya a deal on it if you want it (I just don't use it much).
Here is a "walk around the zoo" that I shot with the Bigma one day... except for a couple of crops, these are unedited JPEGS strait from the camera. No sharpening applied, all shot handheld.
These pictures are nothing special, I simply shot them to show people what this particular lens can do.
BIGMA all 60+ photos (clicky)
(unsharpened, unedited JPEG, backlit, 500mm, handheld)
(unsharpened, unedited JPEG, 370mm, handheld)
(unsharpened, unedited JPEG, backlit, 100mm, handheld)
(unsharpened, unedited JPEG, backlit, 500mm, handheld)
Here's what you get with 50-500 range:
The first is 50mm... and the second is 500mm just a few seconds later as the calf grazed past the mother, shot from the exact location (again handheld, unedited JPEGs):