Like thanhvan said, it depends on the "player's style"
However, here is my preference. Max Mental Strike = useful skill against quest monsters (without element), good for leveling at lower levels, good for when you don't have the super effective element for a monster.
Then, when you reach level 25, you can max the 2nd level earth skill. I choose earth because the strongest earth spell is at 2nd level, not third, and does not require the 7 skill points for the 2nd level skill. From there, it is your choice. Choose one other elemnent (for my mage I chose lightning), and max the third level skill. Now that you have mastered 2 elements, you are good to go, using which ever element is super effective on the monster, and in cases that you don't have that element, you use mental strike. When you reach about level 45-47, you should have about 50 skill points, which you can use to master a 2nd level skill of a THIRD element. I would suggest choosing wind as the third element because the 2nd level skill for wind has a very useful effect; it pushes monsters back. Lightning, Water, and Fire are all acceptable and are good choices for the 2nd element (which will have it's third skill maxed), and Earth is probably the best choice for First element, because when you reach level 25 and can master earth, you will be able to kill giggleboxes.
In case you are wondering, no, there are no elements that are "more common" in monsters. However, if there is a mob that you plan on spending a lot of time on, whether it is for drops, questies, or just because you think it gives a lot of exp or something, then find out its element, and max it.