Probably it is true that once in a while, you could get a lemon. But I suspect that the frequency that this occurs is exaggerated. There are a lot of Sigma haters out there, and not without reason. Years ago, they were quite crap. However in recent years, I think they've improved quite a lot, but some of the more experienced ones will still stick to their original opnions about Sigma.
So far, I've got 2 Tokinas, 2 Sigmas and 2 Tamrons and they've all been good. Mount the lens on the camera and take shots at the extremes of focal length, at wide open aperture, and maybe at f/8. Zoom in on the preview screen and you should be able to see how soft or sharp the image is.
But back to your original question: Sigma 70-300mm f/4-5.6 DG APO, without a doubt. (Must go for the "APO" version!) I have one, and I love it.
DG is Sigma's designation for 35mm lenses that have a special coating that optimises its use on digital SLRs. Can be used on both full-framed and APS-C sized sensors.