If you are thinking about saving gold, choose the one that you spend the most on imbuing. In the game world I play, the most expensive is vampirism (life leech), followed by strike (critical damage), and finally void (mana leech).
If spending on imbuements is not a problem for you, choose the imbuement that is most important to you. If you are hunting, for example, and your imbuements run out, which one will you miss the most? In my case, I would say that strike (critical damage) is the most important. But this can vary according to the level and vocation of each player.
Now if you want to consider only the "numbers", as Anubora said in the comments, void (mana leech) would be the best.