I don't know what to say...but I don't really find rshaman the top healer ( talking about 1.9-2.1k bracket) I find resto druids way ahead of resto shamans. They can just hot up from behind pillars, and displacer beast when they get in trouble. Really annoying class to beat xD
I'd say holy paladins are the weakest healers because they are so easy to cc ( from a mage pov )
So mine goes like this : rdruid>rsham>priest>hpal>w/e is left
Your point of view is obviously biased since you only see it from mage point of view.
Holy paladins got very great tools, they are the healer that have to cast the least, can rotate the best output cooldowns, got the strongest non-DR CC rotation, and got a lot of oh-shit buttons to counter cooldown based setups. The general removal of blanket silences is only a buff to holy paladins.
The general rule of thumb is that if you are fighting a priest or warlock, a shaman will be your best choice of healer. If you are fighting a resto druid, resto shaman will again come out the best, and if you are fighting a mage, you don't want a holy paladin on your team (assuming you aren't playing double healer that is).
With the nerf to glyph of purge, warlocks, resto druids, blanket silences, and mages, it will be a lot more obvious that holy paladins are in fact a top healer (assuming the changes to 5.2 carry through).