This can depend partly on what battery device you'll be using the ProTank and IBTanked cartotank on, and also upon personal preference of the quality of vapor you prefer.
A fixed voltage device can use either standard (2.0 - 2.5 ohm) or low (1.5 - 2.0 ohm) resistance. SR resistance will provide a slightly normal to cooler vape; LR will provide a slightly warmer vape.
If using a regulated device such as a Provari where you can adjust the voltage up or down, either a standard or high (2.5 - 3.0 ohm) resistance works best. Again, choose based on personal preference. I prefer a cooler vape myself, and prefer higher resistance. A 3.0 ohm single coil will allow using voltages between 3.4 - 5.0 ohm safely.
The following safe vaping power chart helps new vapors visualize what ohm to choose for the voltage range that they are likely to use: