For me WHM is the most fun, only due to 5 things. Presence of Mind, Thin Air, Benediction, The Lilies, and Holy spam.

The other healers feel like I am hitting with a wet noodle, especially in solo content. SCH was a big favorite of mine simply due to it having skills from SMN which made it fun and active. Since it was gutted, I have put it down. AST is good but not as fun as WHM.

In normal and casual content there is not enough outgoing damage to make you feel like a healer, and at the same time there is far too much downtime to where you spamming 1 button makes it boring.