The 'show' has the main stress. The 'off' is still stressed - it isn't unstressed - but less so than the 'show', if that makes sense. Like 'photograph' - the 'pho-' and the '-graph' are both stressed, the '-to-' is unstressed. This is for the noun.
The verb 'to show off' the two words have equal stress, like most phrasal verbs.