"shéi" and "shuí" are all correct. You can use both of them to talk to others, and people can totally understand what you mean. But there are more Chinese pronounce "谁" as "shéi". "shuí" as a pronunciation is kind of out of date, but which does not mean it is incorrect. On the other hand, if you are going to take a Chinese exam, or Mandarin exam, or type this character, using "shuí" would be nicer.