Among ASUS GTX970 Strix and the 4 MSI GTX970s.
In this case vaultnoob is correct, the Twin Frozr is the better slightly better one but if you want to go with Strix, this card is also awesome.
The differences between all 5 of those are quite minimal.
GTX970 is in general a good overclocker, this is also a matter of luck, if you get a good chip, it can go high.
The factory stock speed of those GPUs is not really important, you can also OC all of them using afterburner.
The unlocked voltage might help a bit for OC but on GTX970, the voltage is not really holding back the OC.
Temperature is the most important stuff.
Thus, the most important thing is the cooling, Twin Frozr is a wee bit better.