Depends on where. For CB, generally Leech is better than having a healer.

For dungeons, you'll get Scyl for free eventually, and she is far and away better than most others. Apothecary is also popular, since it has a low heal CD and a turn boost.

I use Arbiter in pretty much every place I need a healer. She doesn't quite do as well as the others, but she makes up for it in her revive and TM boost abilities.

Apothecary is the go to for newer players as its the one most easily obtained (relatively)

he is really really good and a normal rare, you can get him from ancients or even mystery shards if really lucky

Reliquary Tender is very good too, but she is a void rare so more difficult to get

Doompriest, Mausoleum Mage, Rector Drath, Godseeker Aniri, are some of the better epic ones.

Then there is ofc Scyl, the free 180 day champ, and Arbiter

O.. I have a Doompriest, but I haven't done much with her, because I didn't fully understand her skills as a dedicated healer!.. awesome.

(any advice on building her?)

You'll want Doompriest in gear that gives it a guaranteed crit, since the A1 procs off of crits. Otherwise, you just want speed and survivability. You won't get any damage at all out of it, but you make up for that with the ATK boost.

Farming 12/3 ?? Bellower with Life Steal and immortal FTW

but Athel works GREAT in 12/3 or 12/6  with Life Steal and  Speed  farming food

you want your doompriest as fast you can get it after that you are looking for surviveability with hp/def

later in the game you'll add resist too

forget crit, your Doompriest will do little to no dmg and the ability on its A1 isnt worth sacrificing other stats, speed/hp/def and later resist is all you need on your doompriest

DP is all about its passive , the faster she is the more healing you get

I agree with that. Forget about the crit bonus from her A1. It's given to a random ally, that's worthless.