Now that we have had the proc explained , a real discussion on who should be the first healer in your raid to get the Hammer of the Ancient Kings can arise.
The basics of the proc is this: any heal, as long as it is not 100% overheal, generates a 10% chance for the proc to occur. Once the proc occurs, and the healer gains the Blessing of the Anicent Kings buff, the the shield absorbs 15% of the total amount (including overheal) of healing. So there are two parts to the question of who should get the mace. 1: who can get it to proc the most? 2: who can put up the biggest shields.
The most important thing is getting the proc to occur. Because of this, many have been saying that Druids, with their use of Wild Growth, will get the most procs, and the mace should go to them. But, what many non-Paladins might not know (and many Paladins as well) is how much Judgement of Light heals for in 25 man raids. Besides your spam of choice (FoL or HL) your Judgement of Light will be the spell that has the most effective healing behind it. Let us compare the two spells.
Judgement of Light Unleashes the energy of a Seal spell to judge an enemy for 20 sec, granting attacks made against the judged enemy a chance of healing the attacker for
Wild Growth Heals up to 5 friendly party or raid members within 15 yards of the target for 1442 over 7 sec. The amount healed is applied quickly at first, and slows down as the Wild Growth reaches its full duration.
Think of the opportunity for the proc to occur with Judgement of Light. Every time an attacker, ranged or melee, who has lost even the most miniscule bit of health hits the judged enemy, the proc will have a 10% chance to occur. Given that everyone in the raid is hitting the boss besides the healers, this amounts to about 20 characters giving you a chance for the proc, multiple times a second. Wild Growth effects only 5 characters, and can only proc once per second. On top of the Judgement's heals, you have the chance to proc on any of your "regular" heals. Oh yeah, and it looks as if your beacon heals count, so your normal heals have twice the chance to trigger the proc.
Once the proc occurs, it is obvious that Paladins can create the most absorbtion from the shields, with the mammoth amount of healing done from Holy Light. And since overheal is no factor in the amount the shield absorbs, Holy Pallies can quickly produce the strongest of shields. Couple that with the fact that any Holy Paladin worth his weight in plate will have Glyphed for Holy Light, and all the melee should be getting a shield as well. Some quick fuzzy math. In 15 seconds you can get off about 10 Holy Lights, at a conservative estimate of 15,000 each, equals a shield with an absorption of about 22,500 damage (even though 20k is the max absorbtion) for your direct target AND your beacon (remember once the proc occurs, overheal goes into the shield's absorbtion) and about a shield absorbing 2,250 points worth of damage for anyone getting the effects of the Glyph of Holy Light. And that's just from Holy Light!
So with apologies to Druids, and without apologies for any bias, that epic mace should find its way into the hands of your raid's Holy Paladin, all other things being equal.