As protoss, in a 'normal' - with no early rushes- I like to go 3 gate Robo and push out, with an immortal or two (if I see Roaches), or just gateway units otherwise. This normally works well and I expand behind the push, sometimes just before, and can often do a lot of damage to a Zerg's base. However not always and if I do not do sufficient damage I end up losing.
When I hit the zerg base - say with his natural expansion up and running - what should I target as a priority to deal the most damage? I normally only have small window of opportunity before mutas pop or he gets enough zerglings to surround me.