I'm surprised no-one has mentioned Sinestra yet! Though we didn't kill it pre Firelands, I thoroughly enjoyed it from a healers point of view.
(In no particular order) Honourable mentions to Spine HC, Professor Putricide, Sindragosa, Madness HC (our first kill my co-healer dc'd like 4 times, good times), Feng the Accursed, Garalon, Blade Lord, Wind Lord...
Probably more, but can't remember