Canon 70-300 IS.
OK, it's outside your budget, but what's money for anyway?
Once you've used an IS telephoto lens you'll wondered how you managed without, and this one gives good image quality even at full aperture.
The only drawbacks are: no focus scale, and it isn't full-time manual focus - meaning that you can't tweak the focus ring manually while in AF mode.
Apart from that, I'll give it five stars.