fire, since at high lvls i need all the damage I can get with destros scaling.
also the least resistances I think.
Lightning does more damage when you spec higher thanks to disintegrate. It's also easier to land lightning spells because they are instant. Missing an incinerate because something moved is not cool.
I think the master level shock spell is the only master destruction spell worth using.
None of the master level spells are worth using, they do less DPS than expert level force you to stand still and you can't chain stagger with them.
Shock is the ideal damage type for a couple reasons:
1) Disintegrate outright kills things low on HP, whereas fire causes things to flee(very bad) and frost paralyzes(not bad but dead > paralyzed)
2) Instant travel time, no waiting for something to connect or having to predict movement, just blast the [censored] out of stuff.
3) It's extremely rare to find something resistant to shock, whereas fire and frost are much more commonly resisted(especially with dragons).
I only invested in shock when I remade my character as the other two fall way short of it. Frost major advantage is slowing and draining stamina but when I'm chain casting TBolts they are stagger locked so slowing and draining stamina are pointless. Fire supposedly does more damage but because of disintegrate this isn't true. I do play on Master so YMMV.