For the first four years I played WoW, my main was a DPS Warrior. I was good at it and got into pretty high end raiding (top 25 US). My second main or "main alt" was always a Holy Paladin for that four year stretch. I enjoyed it, but not as much as DPSing.
Around the middle of Cata I leveled a Priest - and I loved it. My main is now a Disc Priest. I've gone full healer mode now while my warrior gathers dust, and have leveled every healer class at 90 with the intention to heal on them, but didn't enjoy a couple of them so I tried other specs.
In terms of personal preference, not performance, I'd rank them
Priest (Main)
Shaman (3rd most played character, second being my Mage, my only current DPS character. Considering main swap to Shaman for WoD if it makes sense for group comp)
Druid (Level 90 Boosted, playing him quite a bit, up to 539 ilvl)
Paladin (Converted to Prot, mostly retired though)
Monk (Converted to BM)
I enjoy atonement, prayer of healing, and the level 90 talents for Priests. For Shaman, Healing Rain, Chain Heal, and their array of cooldowns. Efflo, exploding mushrooms, and tranq does it for me as Resto Druid.
I do miss blowing up DPS meters occasionally, but I get my fix on my Mage.
Healing is far more relaxing IMO and less competitive than DPS.