when the head of sigma passed leadership to his son, unlike what usually happens, the son put a major emphasis on improving quality. in decades past, they often did not address known design or manufacturing flaws. all that has changed now.
from sigma, i have two dx zooms, one long fx zoom, a fx micro and two ART fx primes which i am very happy with. much happier after re-calibrating the focusing which is important at f1.4. fortunately, the lenses that cannot be calibrated did not need to be.
(also, sigma will re-calibrate for free up to 5 years after purchase, $50 after that.)
reason for mentioning is that nikon also has had focusing issues, too, but no tool for you to fix it. i have read nightmare stories about nikon doing nothing about rear focusing lenses.
sigma and tamron are coming on extremely strong. for that matter, my korean samyang 12mm is quite amazing, too. optics are better in many cases than nikon.
i would not buy older sigma lenses and highly recommend most of the newer ones. the models from years in between, it seems to depend on the model and i have no idea were you can get info on them except a trusted camera store. you could check older issues of NPhoto magazine which does excellent lens reviews.
for newer lenses, i like these folks: LenScore™ Digital Camera Lens Rating but lately, it has been from time to time.