My inclination would be one of the following:
Option 1 (Offensive):
Fire Ball: Malice of the Corruptor full set
Rain of Fire: Ragnarok, skipping the Damage (you'll get 90% damage enhancement, if you really want 95% get some boosters or add a Dam/Acc/End from Detonation in the 6th slot)
Fire Breath: Positions Blast, skip the Dam/Range (but if possible add a generic Range in the sixth slot)
Option 2 (Defensive):
Fire Ball: Ragnarok, skipping the Damage or the Damage/Recharge (see previous comment on boosters although with Fireball you can probably just manage with lower recharge, possibly add a Positron's proc in the 6th slot for damage)
Rain of Fire: Positions Blast, skip the Dam/Range and add a Recharge in the sixth slot
Fire Breath: Malice of the Corruptor full set
The BIG question is where you want procs. Offensively you want a damage proc in Fireball since You'll be spamming it a lot. If I've done my math correctly (which I don't guarantee) the MotC proc has a higher proc chance than Positron's Blast so it makes sense to go there. Rain of Fire, on the other hand, will only have a chance to proc every 10 seconds with a standard proc but the knockdown effect form Ragnarok helps keep enemies in the fire to maximize damage.
Now if you feel you want some more protection moving the Ragnarok set to Fireball makes sense. You'll be spamming it a lot so it will help keep foes on their back. In that situation I'd go with Positron's + Recharge in Rain of Fire since the Procs Per Minute mechanic really goes against patch powers (2 x 35% procs with Positron's versus 75 x 5.1% procs with Malice).
I put it in Fireball personally because I will often cast it even at single targets due to its animation speed, and on most of my builds Fireball recharges faster than 10 seconds, which is the speed of the Knockdown proc when slotted in Rain of Fire.
I have a Fire/Rad Corruptor that I'm considering upgrading from "budget build" to "pulling out all the stops". I built him for Ranged Defense and Recharge with the build goals being Perma-AM and softcapped Ranged Defense. So I'm going to switch out a few sets of Positron's Blast with a set of Ragnarok and a set of Malice of the Corruptor. I know Malice of the Corruptor might be better off in an ST power, but I don't want to lose the Ranged Defense bonus from a Thunderstrike set if I can avoid it (I'd rather stack the RD bonus from MotC then replace a TS bonus, in other words.)
So my question: Between Fire Ball, Fire Breath, and Rain of Fire, which power should I put the Ragnarok set in and which should I put the MotC set in? How does either sets proc work in Rain of Fire?
Thanks.
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