Which element charm is the best to use?

none is better than another

it all depends on what you prefer
constant dmg or unconstant with high dmg and low

from the more constant to the less:

Ice - Earth - Fire - Wind - Light

usually lightning charms are the cheapest.
i wouldnt recommend them on hc, as they are so inconsistent.

Guide :

WHICH CHARM TO CHOOSE
Okay. It wouldn�t be a guide without this. The three types of charms are infamous, as well as their benefits and drawbacks, I�ll just say this.

Ice-Most consistent damage. This is the preferred, I believe, as magicians depend on consistent damage to kill. Ice charms provide good damage, consistent damage, and you can count on them time and time again. I suggest Ice Charms if you can only get a few of the important stats listed above

Fire-Less consistent than Ice, but can cause more damage. I prefer fire charms, if you can get some Dexterity on your items, there�s no real reason not to choose Fire. Fire charms cause some damage, they�re fairly consistent, you won�t hit for 100s and you can end battles just as fine as with Ice Charms. I suggest Fire Charms if you can get most of the important stats listed above

Lightning-The backsheep, if you will. If you don�t like hitting 0s with a sword in melee, you won�t like casting for 0s with a lightning charm. You may change your mind when you cast with your lightning charm and you do more damage than Ice or Fire can do. I suggest Lightning charms if you�re able to amass a large amount of Life per Kill, Life per Combat Round, and Max life on your items.

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Enhanced Effect on your charm is your savior. 50% will be fine for 1-10. From there, try to get 100% on your charms. Level 25/30 is when you should try to start getting 150% EE + on your charms.