I am planning on building a system with dual 7970s. I can't find any site that gives a straight anwser about the best waterblock, or card for over clocking. I would prefer to have a water block that allows me to see my coolant, I think that kind of waterblock has a specific name but I'm still a n00b so I don't know.
So you want a waterblock with an Acrylic top to see the coolant (For example like this )
The main issue your gunna have is waterblock compatibility. Most fullcover blocks are designed for reference based PCBs. EK have a useful site that allows you to search for GPUs and it will tell you if they are reference based or not (http://www.coolingconfigurator.com/)
Your not going to find much information in the way of "best waterblock" as testing and reviewing waterblocks can is time consuming and messy.
I personally like the EK water blocks (the older design without the silly circles on). I have tried an XSPC one a while ago, but that was when i was using just a GPU block on a 6870. Not a full coverblock.
Hope at least a little of this helps. :P