Currently the strongest PvZ strategy you can use. I have not looked to find an optimal build order for this strategy, but basically it is mass void rays that maxes out 6 colossi with some gateway fodder.

1) 3 gates + stargate
2) expand
3) +robo & 2nd stargate
4) constantly pump out of both stargates (void rays)
6) Get 2-3 more gatways once your minerals start to exceed your spending
5) Build colossi off one robo until you have 6 colossi.

That is the generalized build that you will want to use (re-order as you see fit). How this works:

Basically, you just sit at two bases and defend yourself. Keep your army at your main until you hit the critical number of colossi and have a large fleet of void rays. The colossi will tear apart any ground forces, the void rays will beat any air units, and the small number of gateway units is just enough to keep the zerg army from attacking the colossi. Some additional points to keep in mind:

1) Scout and build the gateway unit to build their composition (i.e. zealots vs ground, stalkers vs corruptors)
2) Deny excessive expansions for zerg with raiding parties of zealots (keeping your main army at your base to defend).
3) +1 weapons is delayed, minimal ground upgrades
4) +1 air attack early on, +1 air armour later.
5) Avoid too much tech as colossi + void ray is already lots of tech.