From a PvP point of view (ie from a Protoss perspective, even though you never really see this in a 1v1 setting):
Mmmm the problem with mass Void Rays if you let their numbers build up is that they kill your units so quickly, since you have fewer units than the other races, that you'll very quickly run out of things that shoot up in a balanced enough army to deal with their Zealots too (Sentries, Zealots of your own, Collosi).
Storm can be interesting if they don't magic box at all, but if they do you won't hit much, and storm templars is already a hefty investment in PvP where they're mostly useless in general.
What I'll instead suggest is Archons. Yea, you heard me! 5-6 Archons with one Mothership can clear out every single Void Ray in 3-4sec, while losing almost nothing. Massing Archons will leave you with a lot of excess minerals which you can sink in zealots to counter his Zealots, and after the Void Rays fall, Archons can help with splash damage on his Zealots too.
Of course we're talking about late game mass Void Rays, in smaller numbers Stalkers can shoot them down before they become a threat. Make sure you scout to see what he's doing, churning out a Mothership can take a while, but it's way worth it.