1. WHM+~SCH~
2. AST
3. WHM

From what i see, around 70% of healers running around on my server are WHM. 20-25% are SCH, and the rest are AST(there are more AST, but they have switched to either WHM or SCH until issues are fixed).

Then,i dont ever dare to, but i would say WHM is the "best" if we're talking about healing, getting people as high up in health as possible fastly and just doing the pure healing work. Still though, i can't say WHM is better than SCH, they are both very powerful healing classes with very nice tools available, and they perfectly complement one another, square has done amazing work in balancing these two healers over time in this game.

For the rest, from what you see in threads here, AST is currenly considered underpowered, and square is taking a look into it. Content is doable with AST, but it's much less safe than doing it with WHM or SCH or any combo of healers with those two.