I remember playing these games back then, but I couldn't get very far in neither of them.
Today, I'd have more patience, but I don't have much time.
Already boght this pack.
Which do you suggest?
Which one's the best of the bunch?

I'd say Ishar 3 is probably the best of the series.
The problem with playing it first is that these games were meant to be played as a series, and that not having a party to import from Ishar 2 will make the beginning very difficult.
However, if you can get past the first few fights until you get enough money to buy decent equipment, then you should be all right.
If it's too hard for you, then start with Ishar 2, that should be easy as long as you remember to go to the village to recruit some party members before you go fight.

My suggestion is to avoid Cristals of arborea. It could be considered a good strategic game, but it's not a good game as Rpg

I personally liked Crystals of Arborea, especially for its need to split my party in two in order to find all crystals before the enemy.
As for Ishar, the second one is clearly my favorite. It's more linear than Ishar III, and thus easily playable without the occasional peek into a walkthrough. In addition, party psychology (likes/dislikes between party members) play a much more important role in II, which adds an interesting twist to the game.