The best lvl reqirement for an enchantment is 50 lvls, However its extreamly rare to get this.
As the for the best enchantment, It depends on personal choice.
You can get Silk touch (Allows you to pickup anything in its original state, Stone stays as stone, glass drops not breaks ect...)
fortune (Increased drops for items, espeshaly diamonds and redstone ect...)
Efficiency (Makes the pick mine faster, so with a diamond pick to startwith, you get almost inta mine)
Unbreaking (A chance that on breaking a block the tool takes no dmg thus making it last longer)
there are a few more but thouse are the main ones.
you need 30 bookcases maximum to get the higher lvl enchantments