Mange tak for det
That's terrible to say to someone.
Unless this friend did and/or said something awful to you. Depending on the situation that makes you say this phrase.
Tak for feedbacken :(
Could this be used with sarcasm?
Come on, we all have one
Hey with friends like this, who needs enemies
If somebody wonders what's the basic adjective form of "værste", it's "slem".