Well, I'd like Holy Nova. Instant cast, check. No cooldown time, check. No aggro, double check. Also damages enemy, check friggin' plus. The only thing Holy Nova doesn't do is get you a beer. Imagine casting an instant cast Medica and Holy at the same time. Having to choose whether to finish casting a heal on a low-hp tank and risk getting killed in a Landslide or Weight of the Land from Titan NM? Now you don't have to! Also you can help DPS Titan's Heart and break Goals while you're at it!

If F2Ps count, there is also Thunderbolt from the Weather tree from Asda Story. Will do 20% damage to ANY monster, no questions asked. Holy smite from Eden Eternal is a ranged Holy(you don't have to get in range of a mob to AoE stun). Also Holy Light from Wizardry Online can potentally 1-shot any undead in the game if you have chaotic alignment, which would help with WP speedpulls. FATE has Shockwave which in the Defender class of magic is an non-elemental AoE knockback.