An "Ohm" is the standard measure of resistance in an electrical circuit. The higher the resistance, the shorter the time between recharges. The more you recharge your battery, the sooner you need to replace it. Without altering the voltage, lower resistance equals a hotter coil in your atomizer. Without altering the resistance, higher voltage equals a hotter coil in your atomizer.
Different resistances are usually achieved by differences in the way the coil is built. For a wire with a particular resistance, more turns on the coil equals more resistance but a larger surface area to produce vapor. The producer can achieve the same overall resistance by using a higher resistance wire (i.e. thicker) and less turns on the coil.
The flavor and vapor output of any given juice changes as you change the heat of the coil.
The flavor and vapor output of any given juice changes as you change the design of the coil.
Differences in coil build plus differences in resistance plus differences in voltage = differences in replacement coil life.
Difference in quantity of recharge cycles = difference in replacement battery life.
"Best" becomes subjective based on how much battery life you need/want between recharges, how much atomizer life you need/want between replacements, and what you perceive to be the best flavor/vapor production for you. What is best for one person may not be (probably won't be) best for you. You will only find what works for you by experimenting.
The "standard" for atomizers is 2.5 ohms. If you are interested in experimenting, I would recommend buying a rebuildable such as the CE5 or the T3 (not a specific recommendation, just a style for you to look for). There are several kits available in the $12 to $20 range that come with the clearomizer and 3 atomizer heads, each at a different resistance. If you want to go the extra mile, get a VV battery.
Please also take a look at the following chart. It puts most of what I've stated here in one simple picture.
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