DxOmark (Click here) tells you which lens is the best for your Sony A99. I guess none here will be surprised to see the Zeiss lenses on top:
The Zeiss 24-70mm lens wins. But the other zooms are pretty close (see table on the bottom of this post)
Too bad DxO had no time to test Samyang lenses. I suspect they are pretty close to the Zeiss lenses.