What is the opposite of a mutation (the regular state)?
A mutation can be both the act of changing the genetic material (the process) or the changed genetic material itself (the result of the process). You're using the second meaning. See What is the definition of a mutation? for more info.

That being said, the opposite of a mutant allele is a wild type allele.

According to Pierce in his book "Genetics: A Conceptual Approach" (2012):


  The normal allele for a character is called the wild type because it is the allele most often found in the wild. (emphasis mine)


The same way, the opposite of a mutant phenotype is a wild type phenotype. 

Besides that, you can sometimes find "wild" alone, without "type": wild allele, wild phenotype etc...



Source: Pierce, B. (2012). Genetics. New York, NY: W.H. Freeman.