So, lets assume the invasions attack the capital cities of Azeroth, all 8 of them (not including post TBC capitals or Shattrath, aka: Faction only capital cities) which one would be the most likely to be destroyed if any?
My, money, personally would be on Thunder Bluff.
Why?
Thunderbluff is a mostly peaceful city, filled only generally with hunters and nomads with very little defences. It has a high vantage point, which would usually work to its favor, but the Legion is invading from the sky, and has a tactic that usually involves setting everything on fire.
And TB happens to be literally the most flammable city, even Darnassus has more water to go around to help put out the fire.
I am also not suggesting this as some kind of bias but logic, TB is the weakest city of them all, the others, possess at least some kind of magic defence, stone fortifications, points of retreat, or points of counter attack.
E.g.
Why not Ironforge? Because Ironforge is a solidly defended fortress with multiple moat bridges serving as a natural retreat point, and the deeprun tram in the event the city needs to be evacuated cause the Legion might attempt to destroy it by caving in the mountain from above.
However, mountains can take a hell of alot of hits before finally being destroyed, and this works unfavorably for Thunderbluff, which in contrast, is a vulnerable site from a skyward vantage that works against it when the Legions general invasion point, is above.
By now, the Exodar is probably operational again and can probably fly, even if it couldnt, it has at least 2 evacuation points, and a nearby harbor for escape and retreat. Thunderbluff has only got elivators leading down to the surface, and zepplins that could come to evacuate, if, they werent being hounded by fire.
If the Elivators were destroyed, TB would be a death trap, with people unable to leave because they'd be grounded, even with wyverns to evacuate, it'd still be a hard pressed escape.
Silvermoon has solid defences against the Fel, though the fact that they've been invaded from within multiple times makes it a risky place to be.
Undercity has multiple layers of defence both above and below, and a point of retreat into Silverpine if they need to run from the Legion.
Orgrimmar's canyons might seem like a weakness, but much like its steel defences, they work as a solid natural barrier from skyward attacks and keep the place mostly unharmed as it would survive many blows from such strikes.
Stormwind has been fortified multiple times and has the largest number of evacuation points, and defences, making it one of the best cities to defend against the Legions attacks due to its access to reinforcements from multiple areas within its own districts.
Thats my take anyway, whats yours?