"Ich mag Deutsch nicht" is perfectly fine. You could say for example: Ich mag Deutsch nicht, ich finde Italienisch viel schöner. "Ich mag nicht Deutsch", however, is wrong and can't be used. Please find additional information on this website: http://german.about.com/od/grammar/a/The-Position-Of-Nicht.htm

Deutsch mag ich nicht! That is the correct way to say it. The other two ways are not wrong, but I can't think of a situation where you would use them.