What's the "best" AoE classes in the game for specific situations?

[Vodomir]

Dragoon - RoT - Potency 100 but only 120 TP cost so is much easier to spam. Doom Spike which has to be in a "line" but thats easy when mobs get clumped up on one tank with a potency of 160. Lastly, Dragonfire dive which is a whopping 250 potency though it can only be used every 3 minutes.

This. Dragoons can spam for 160 Potency every GCD and dmg can further be improved with Heavy Thrust (can be kept up all of the time, you'll just have to sacrifice a GCD to renew the buff) and Blood for Blood (Off-GCD dmg buff). I expect Dragoons to be very competitive in terms of AoE DPS, once the mobs are all clumped together.

So around 140 potency every GCD, whether you are tab thundering for <5 targets, or miasma2 + tri disaster for more than 5 targets.

140 potency per GCD with fancy tab targetting? I guess you missed the point where Dragoons can spam for 160 potency every GCD with the ability to put buffs and off-GCD AoE Skills on top of that.