Depends on the power level you need. Obviously the hit at over 13 Watts was too much, so you need to go down. (Power=Heat=Watts=V*V/Ohms, or, Power in Watts is equal to voltage squared divided by resistance in Ohms)
From the formula for power, it should be obvious that voltage has a very big impact and just taking the lowest power offerings in your post, 5 Ohms, results in 5 Watts at 5 Volts and 7.2 Watts at 6 Volts.
With 4.2 Ohms the 5 Volt battery will produce 6 Watts and the 6 Volt battery will put out 8.6 Watts. I seldom like anything less than 8 Watts, but depending on the
juice I have gotten by with anything from 6 Watts to 12 Watts.
If I were you, I would get ahold of a range of resistances and find which one works best for you and your
juice. Then do the math so you have an idea of what your Watt is so you can reproduce a range near that Wattage with whatever gear you obtain in the future. After that, see if you can get the Ohm you like in a larger quantity to save some $ and have plenty of spares on hand.
