The main word, which literally translates to something like “shifted out of place”, is verrückt.
A close second is wahnsinnig.
In common usage, the former is used to describe something very unusual and the latter to refer to something extremely large or intensive.
The clinical word for “insane” is geistesgestört or geisteskrank.
More Creative Ways
Other words in German are irre and toll (both with the alternate meaning of “great”) and irrsinnig.
Implying stupidity, the words are bescheuert, bekloppt and hirnverbrannt.
Someone who’s lost his mind is übergeschnappt.
In certain German regions närrisch is also common.
In the North mall, the South spinnerts and in the Rhineland jeck can be used.
The verb (as in “talking rubbish” or “being bonkers”) is spinnen. It literally means “to spin”, in the sense of weaving yarn.
Die Party war total verrückt! The party was totally crazy.
Das sind wahnsinnig niedrige Preise. Those are insanely low prices.
Er spinnt. He’s crazy.
Das ist ein tolles Buch. That’s a great book.
Deine Idee ist total bescheuert. Your idea is completely stupid/crazy.