A investigation has showed that Inna's Radiance is always the priority of a Monk. No matter if you are under Paragon Level 48 or above Paragon Level 50, 60% chance that you have an Inna's Radiance for you Monk's Helm in the Softcore mode. Three Helms have accounted for more than 90% of helms part. They are Inna's Radiance, Mempo of Twilight and Andariel's Visage.
Inna's Radiance: You can easily find +180 Dexterity, +40 Vitality, + Critical Hit Chance 6.0% Inna's Set Spirit Stone with around 20,000,000 Diablo 3 Gold. With the same stats, Radiance is much cheaper than Mempo of Twilight. This is mainly because you can get attack speed bonus in the Mempo of Twilight. But Inna's set is always the blessings of Monk. Especially for those who use Wave of Light Build which would consume you a lot of Spirit. You need fast Spirit Regeneration. Moreover, the four set would decrease the spirit cost of Sweeping wind which is the main means use to clear the Monsters.
Mempo of Twilight: This is the second choice for Monk but also the most expensive choice. After all, you can gain over 71 all resistance, Attack Speed 8%, 170 Dexterity, socket and 10% life for certain. There is only one Random Magic property. You do not have to be afraid that you would choose one garbage at the high cost. All the Mempo of Twilight is a treasure. If you are lucky enough and farm a MOT, you are going rich.
Andariel's Visage: This one is only chosen when player is short of gold. Any way, the combination of Attack Speed and Critical hit chance make it better than the rest of helms. Most important thing is it is really cheap.