ништяк is from narco vocabulary, means "high" after taking drugs, may be used figuratively.

пиздец - obscene, literally, vagina-like
пипец - less obscene, but literally, penis.
охуеть - obscene, literally, become penis-like

офигеть - to become like a fig sign http://en.wikipedia.org/wiki/Fig_sign , mildly obscene.

Only отпадно is not obscene.