What will be the best coax cable for QRP?

  • RG-8U
    • pro: least amount of loss.  often used for tower/shack cable run of longer distance
    • con: thickest and stiff.  not for portable use.
  • RG-8X
    • Just a bit thicker than RG-58 but much less lossy (QRP, you want to keep the losses as low as you can... 8X is very forgiving
    • but how much are losing compaired to other factors like antenna, SWR etc)
  • RG-58C/U
    • hicker, but still quite thin
    • less fragile, less lossy and will fit UHF connectors
  • RG-174/U
    • Pro: Thinnest and lightest coax
    • Con: Very frail and cannot take much mechanical stress
    • this isn't too bad for short runs into a perfect match, but RG-58 is better
    • For the short length, the additional weight and pack size of RG-58 is insignificant.

Something to consider though, the matched loss at 30MHz of RG-58 is 7.7dB per 100 metres, the matched loss of RG-8 is 3.2dB per 100 metres. This makes the RG-58 seem poor, but with perhaps a typical cable run length of 10 metres (30 ft) then you're talking 0.77dB loss compared with 0.32dB loss, and a difference of 0.45dB would be completely insignificant. And I'm assuming that as the OP is considering really small diameter cable then he is also thinking about portable operating, in which case the cable lengths could be even shorter. 
I run approx 1 metre of RG-58 and the matched loss on that would be 0.077dB, and even at a 9:1 SWR the loss would still only be 0.1dB so I don't worry too much.
I guess what I'm saying is that the cable loss for short runs, typical for portable operating, is really insignificant and it makes no odds whether you're running QRP or not. You'll still only lose a tiny tiny insignificant fraction of the signal, i.e 1 or 2% that will be completely undetectable compared with the other losses in the system. So, really, one may as well just use whatever cable is practical and readily to hand. 
Even RG-174, with a loss of 18dB or so at 30MHz over a 100 metre run, only has a loss of 0.18dB for a 1 metre length which means it only loses an extra 2 percent compared with RG58.... again, insignificant compared with everything else.