What is the best strretch for a strained neck?

Depends. There are alot of muscles in the neck, so it depends which one is bothering you. Try and move so that you apply a gentle stretch to the offending muscle. If that doesn't work, being seen by a physical therapist would be helpful.
Careful. If you feel like you strained your neck, you would want to be careful not to injury it further by "stretching" it. Moist heat and rest would probably be a better initial alternative. If it does not clear up quickly consider being seen.