For both fun and challenge, WHM is highly recommended.
You will be always on your toes, need to be aware of both enemies and teammates, and know a fight well.
WHM can do some serious DPS aside from healing.
Nice outfit too

Leveled WHM, SCH, BLM and SMN to lv50. Find that ranged DPS classes are easier to play, but can be boring...healer classes are more engaging and demanding, but much more fun too.