DRK gets Flood of Darkness at 30. You can't compare all the tanks' AoE potential just by looking at their AoE skillchains.
Code:
Lv Pot. Action Remarks
DRK
** Darkside Always-on 10% bonus
06 150 Unleash
30 250 Flood of Darkness oGCD but MP-dependent; I don't have average number of uses per timespan available
52 300 Salted Earth 1 per 1m30
56 200 Abyssal Drain 1 per 60s
64 210 Quietus Gauge
72 160 Stalwart Soul
74 300 Flood of Shadow Replaces Flood of Darkness
WAR
** Berserk Forces Crit/DHit for 10s every 90s
10 130 Overpower
40 200 Mythril Tempest
45 220* Steel Cyclone Gauge. When forced Crit/DHit, more like 410
60 250* Decimate Gauge. When forced Crit/Dhit, more like 460.
72 400 Chaotic Cyclone Replaces 1 Decimate per 30s. When forced Crit/Dhit, more like 750
PLD
** Fight or Flight 25% bonus for 25s every 60s
06 120 Total Eclipse
40 220 Prominence
50 295 Circle of Scorn 25s
72 375 Holy Circle Replaces 2.5 Prominence Combos per 60s, a gain of 1025
80 1,200 Confiteor Replaces 1 Holy Circle per 60s, a further gain of 825 per 60s
GNB
** No Mercy 20% bonus for 20s every 60s
10 150 Demon Slice
40 250 Demon Slaughter
62 650 Bow Shock 1 per 60s
72 320 Fated Circle Gauge
Rough AoE Weaponskill potency per minute at Lv30
DRK: 150 / 2.5 * 60 = 3,600 (3,960 with Darkside)
WAR: 130 / 2.5 * 60 = 3,120 (3,575 with Berserk *assuming 1.5x crit multiplier)
PLD: 120 / 2.5 * 60 = 2,880 (3,180 with Fight or Flight)
GNB: 150 / 2.5 * 60 = 3,600 (3,840 with No Mercy)
Rough AoE Skillchain potency per minute at Lv40
DRK: 150 / 2.5 * 60 = 3,600 (3,960 with Darkside)
WAR: (130 + 200) / 5.0 * 60 = 3,960 (4,538 with Berserk *assuming 1.5x crit multiplier)
PLD: (120 + 220) / 5.0 * 60 = 4,080 (4,505 with Fight or Flight)
GNB: (150 + 250) / 5.0 * 60 = 4,800 (5,120 with No Mercy)
In terms of Floods of Darkness, DRK is behind GNB by 4.22 Floods of Darkness per minute at Lv40. I don't have exact numbers on how quickly DRK refreshes MP, but as Flood of Darkness only costs 3/10 of a bar, this doesn't sound like a problem since by Lv40 they have Blood Weapon to restore MP every 60s.