All based on player skill really, and also on the fight as various fight mechanics make certain healers betetr on certain fights.
Pally - Essential IMO to have one of these in your raid as they are the best tank healer without question.
Druid - If played properly it's hard to beat a druid on raid heals.
Holy Priest - Great heals all around really with a lot of instant casts. They are about on par with druids for most raid healing fights.
Disc Priest - Decent tank healers if played properly but very hard to judge how effective they are due to shielding and whatnot. I honestly have not have great experiences with them and in a 10 man I prefer to fill the tank heal spot awith a Pally.
Shaman - Good all-around healer so long as you don't just spam chain heal all day as this is not the best way to play. Cannot keep up with druids or priest though from what I have seen on raid heals but do provide some great buffs and heroism.
My favorite 10 man combo is Pally, Holy Priest and Druid/Shaman.