Zhonyas and Guardian Angel

Which item should I get and in what situation should I get that item?

do I get zhonya's or Guardian Angel if I am being bursted down?

You get GA instead of Zhonya's when you're not using an AP character.

if i am specifically playing an ap character like evelynn or diana which would i get first?

It depends. If you're an assassin with burst, Zhon is best assuming you live in most confrontations; it'll help reduce CD's, dodge enemy attacks, and give you more AP. If you're dying or you're just plain squishy, then GA would probably be better although you miss out on the AP.

ah thank you just needed to clear it up on which would be a better choice on the situation as an ap champion