Well its very good, and for the price of course the value is much higher than the manufacturer zooms. Of course this is comparing it to other zooms and not primes.
But best ever, well that is an overstatement. Perhaps best value zoom ever.
Lets look at the 28mm result from the photozone test of the Tamron
The Nikon 24-70 is basically breaking the chart here, and is being limited by the 10mp sensor. At 24mm vs 28mm at f2.8, the Nikon is sharper in the borders than the Tamron is in the center, not only that, the nikon is sharper in the center at f2.8 than the Tamron is at any f-stop. Is that what they call 'ownage' ?
Unfortunatly it has only been tested for Nikon so thats the only one I can compare it to. But at least it shows its not the best ever. You still get what you pay for, even tho the returns are diminishing.
The Nikon lens is just insane, sharper wide open than any of the Nikon 24 or 28mm primes, thats crazy, the point of buying a prime is lost with that lens.