I've recently been round this loop, although admittedly on the Canon side rather than Nikon. Fortunately, The Digital Picture did a very nice comparison of most of the available 24-70 lenses:

• Canon (both original and II)
• Sigma (not quite sure why you're not including this one in your possibilities, as it seems to be available in F mount)
• Tamron

The primary conclusion is the expected one: the Canon lens is the best, particularly in the corners on full-frame. But then it should be as it's twice the cost of the rest. I'd imagine the Nikon 24-70 is pretty much up there with the Canon in terms of image quality. The other notable point is that the Sigma is reported in multiple reviews as being very poor at 70mm f/2.8 - if that's important for you, you probably want to avoid it. Unfortunately, The Digital Picture haven't done a review of the Tokina, but it is generally reviewed as being a very sharp lens, but beware about the focus motor.

For what it's worth, I went for the Tamron in the end: a combination of the image stabilization and a good price from a well known Hong Kong retailer was what made the decision in the end. Am I happy with it? Yes - it was bought to let me do some gig photography, and lets me do that. I personally wouldn't use it as a "walkaround" lens though - it's too big and heavy. You may be more prepared to put up with that than me though!

As for how you make the decision, to me it's a question of what you want the lens for. If you need that corner sharpness, then the first party lens is what you need. If you don't, it's a toss-up between the Tokina and the Tamron. The Tokina is probably sharper, but the Tamron has quieter focusing and VC. I'd probably avoid the Sigma (due to the poor performance at 70mm f/2.8 - why are you buying an f/2.8 lens if you're not going to use it at f/2.8?) and the cheap Tamron.

Two final comments:

• Everyone is waiting to see what the Sigma 24-70 Art is like - due out real soon now. If it's anything like the quality of the recent Art lenses, it will be a real competitor with the first party lenses - but will probably also have a price tag to match.
• I wouldn't trust anyone other than Lens Rentals if they say "you can get a bad copy": firstly because what is a "good copy" on one body can be a "bad copy" on a different body, and because you need to see more than one copy of a lens to know if it's bad QC or just you being unlucky.