If you are using a standard fixed voltage eGo style battery that gives out 3.3 volts, then I would look at the 1.8 ohms.
If you are using an eGo style battery with a higher fixed voltage like the Evod which outputs 3.7 volts, then go with the 2.2 ohms.
If you are using any variable voltage/variable wattage style battery then go for 2.5 ohms.
On my mini protank, I get good performance between 6 to 7 watts and that's what you can get with the setups listed above.
Getting too low a resistance coil with too high voltage will give you a burnt test a lot of the time.