The best skill for a Sorcerer is the Crystal Fragments skill, this is one of the greatest skills for all Magicka Sorcerers and is used in pretty much all the builds as main or semi-spammable. Of course there are plenty more good options, it is always recommended to take a look at the skill setup of each Sorcerer Build individually. Not every Sorcerer build does have the same skill setup, it all depends on the activity you are trying to optimize for.