many look at all te healers in a different way in WoW.
Paladins are considered best single target healers, they are very mana efficient and have strong bubbles and buffs. can also cleanse many different debuffs.
Druids are the kings of HoTs, they have 2 instant hots one of which stacks three times, and if they spec as a tree can be very mana efficient, they lack bubbles but have Nature's swiftness and and Swift Mend, both strong ways to save a tank in need with bigger heals. As well as Innervate and Battle Rez which are strong tools in a raid environment.
Shamans lack hots and bubbles but bring strong AoE healing abilities to a raid group as well as strong buffs to increase any groups performance in the form of totems. Shaman can be Main Tank Healers, but truely shine in a 25 man raid spamming Chain Heal to top of there group. They also have nature's swiftness and earthen shield to help with healing on the main tank (or whoever may need it for the situation they are in) and also Mana tide to refill caster groups mana in need.
Priests are considered the jack of all healer, they have big heals (pallies and druids), they have bubbles (pallies), they have hots (druids), they have AoE heals (shaman), and they can regen there own mana (shaman and druids), and can cleanse pretty well (pallies). They are not as effecient as pallies or druids, but have a large pool of spells to get the job done. they also have fear ward which can be very helpful on certain encounters for tanks.
so basically it depends on what you want but here is the broken down version:
Paladin: Main Tank Healer Buffer
Druid: Raid Healer (hots) and utility
Shaman: Raid Healer (AoE heals) and buffer
Priest: Gap Filler Healer
So to answer your question in my oppinion each healer is great and each has a purpose in a raid.