If you are snowballing, you go for deathcap first beacause the AP allows you to clear waves faster and gank more. Zhonya if you need defense.
Lichbane first is either you are extremely ahead but didn't have enough for a large rod first back and went sheen instead. Or you can safely poke at your lane opponent. Or if you want to splitpush, or if you are really behind and just want an item to teamfight with.