This is a hard question because there have been so many raids and dungeons that I've enjoyed over the years. I don't have any screenshots to share, but I'll try to explain the reasons I liked the raids/dungeons I did.
I can't really choose one raid/dungeon over everything else, but I can narrow it down by expansion.
In Burning Crusade, I'd have to say my favorite dungeon was Heroic Shattered Halls because so few pug groups could get through it. I love being challenged as a healer and that dungeon offered some of the best ones.
My favorite raid in Wrath? ICC, hands down. I loved the boss mechanics, the graphics, and the challenge that some of the heroic versions presented for healers.
In Cataclysm, I'd go with Firelands. The reason is that I liked the feel of the raid; the environment really fit the name of it. The symmetry appealed to me and Heroic Alysrazor was my absolute favorite fight to heal.
This tier? I'd say SoO, because it has an open feel I like and a lot of bosses have mechanics that don't feel rewashed. All of the fights require a healer to be flexible and in terms of difficulty, I'd say it's probably one of the harder tiers for inexperienced healers that are prone to panic easily. Since I love being challenged as a healer, SoO is my favorite raid this expac by a wide margin.