Whats the best 7870?

Overclocking potential is never an absolutely certain thing with any card. The HAWK version does have higher quality components that are designed to facilitate better overclocking, so it has a better chance of achieving a better overclock than most of the other cards you listed. The factory overclock given to the cards can be used as an indicator of how far the manufacturer thinks the card can overclock. The ASUS card has a mere 10MHz overclock, while the Gigabyte and MSI HAWK have 100MHz, the Twin Frozr II MSI card has a 50MHz OC, probably because the Twin Frozr III cooler isn't as good as the Twin Frozr IV on the HAWK version.

Then again, the 7870 was pretty clearly underclocked at stock, pretty much any 7870 is going to be a decent overclocker, a card like the HAWK version is better if you really want to push the card to very high levels. If you are after a more modest overclock, a cheaper model may be better if you want to save $10-20. The Gigabyte OC and MSI HAWK have the best out of box performance, a cheaper model like the ASUS may be able to achieve a similar overclock, or it may not, it's luck of the draw.