WHM's been the most popular (in terms of overall created numbers) Healer since ARR.

I think specifically that Suncatgirl White Mages are the single most popular race/tribe/gender/Job combo in the game, at least if I'm remembering my census right.

Doesn't help that they're also the only Healer with an actual useful Job mechanic and the only Healer that didn't get gutted in the SB->ShB transition, but they were wildly popular before that.