what do they cast in order to recover the most HP?
OK. Let's consider some real game applications.
Your 4th level party is in a fight with a Hill Giant. The Hill Giant hits the fighter doing 18 points of damage. The fighter only had 10 HP left. The fighter falls to 0HP and goes down.
A goodberry would bring him up to 1 HP
A healing word would bring him up to 1d4+3 HP (let's say 6)
A cure wounds would bring him up to 1d8+3 HP (let's say 8)
A healing word (II) would bring him up to 8
A cure wounds (II) would bring him up to 12
If the Hill Giant hits the Fighter again, the Fighter will take 3d8+5 damage (18 likely)
Now I would say that in this case, healing of 1 hp and healing of 12 hp is no different. One hit and the fighter goes down again.
However,
if we apply your logic, Cure Wounds II is 12 times better than goodberry because it can bring the fighter up to 12 HP instead of 1 HP
I had a Ranger(Hunter)/Rogue(Arcane Trickster) that was a far better healer than the party Cleric. I used Arcane Trickster to access Find Familiar (got a bird) that delivered goodberries to downed party members.
So think about this. 10 times an ally could go down due to falling to 0Hp. 10 times I could bring them up (as well as stabalize of course). The range was about 60 feet for one round. This required neither an action nor a bonus action. Concentration was not required. It cost one first level spell slot.
And you then ask "Which do you think is the best in-combat healing spell?" and then you ask "Which spell do you think recovers the most HP?" as if it's the same question re-worded. It's not, and goodberry is far and away the best in-combat healing spell until at least mid level. Better than Healing Word, better than Healing Spirit, and WAY better than Cure Wounds, and it heals 1 HP.
Because every cleric or paladin I've ever seen, at least before getting to high levels, has always resorted to Cure Wounds or Healing Word. If there's another healing spell, then I don't know about it.
Thats because Druids and Rangers are better healers than Clerics and Paladins.
Here's what Clerics and Paladins are best at: Doing Damage. Having the same classes the best at doing damage AND healing would be crazy!
No, I agree that Find Traps is a bad spell, and that it would be better if it had been Locate and Disarm Traps. That goes back to the previously-mentioned situational analysis of resource efficiency. You're usually better off not worrying about the trap, and using the spell slot to heal some of the damage, rather than casting Find Traps to possibly learn about a trap that you can't locate or remove.
You missed the point. The point was that comparing Healing Spirit to Prayer of Healing is pointless if Prayer of Healing is a terrible and useless spell, even if Prayer of Healing was the only spell that was supposed to be used for out of combat healing.
Unless you're arguing that all existing out-of-combat healing magic is worthless,
Seriously? This sounds far fetched? I've been saying it over and over. YES. I am saying that the previously existing out-of-combat healing magic is worthless.
The only out of combat healing that was worthwhile was resting. Short rest/Long rest, either way, I would suspect far more healing is provided by resting in the game than by any out of combat healing spells. I suspect this holds post-Xanathar's as well.
though, then I can't understand why you'd want to introduce a vastly-superior spell that isn't even allowed to the main healing class.
You are mistaken. Druids were the best healing class prior to Xanathar's, and the best healing class post Xanathars. In fact, with Circle of Dreams Druids - it's not even close.
Though admittedly, Life Cleric/Circle of Dreams Druids are monster healers.
This isn't 2nd edition anymore. Now Clerics do more damage than Druids but Druids are better healers. (With the exception of Moon Druids from levels 2-4 - but that really is bad design)