Seems most healing classes have some recourse or another (bard gets to choose which spells) for
- one turn single target heal
- one turn multiple targets heal
- multiple turn single target heal
- multiple turn multiple target heal
Assumptions:
- All spells calculated at spell slot 9
- Only spells are considered and no class features (paladin lay on hands compete with Heal for single heal at maximum paladin level)
- Only core bard spells are considered
- No other utility benefits are considered other than healing
In general, Mass Heal leads the chart across all situation... though you only get one casting of that. Heal followed by cure wounds for one turn, single target heal. For Mass healings... the Mass Cure Wounds take the lead.
If ignoring the number of turns in which the spell would take effect, a few notable stand out; Prayer of Healing, Aura of Vitality and Conjure Woodland Beings (Dryad).
Aura of Vitality doesn't scale but provides a total healing capacity (one target or multiple) over all the other spells except Prayer of Healing at 3rd and 4th level spell slot being bumped out only by Mass Healing Word at 5th level spell slot.
On the flip side, Druid has access to the slightly less potent Conjure Woodland Beings (Dryad) which summons two Dryads that have each Dryad casting Goodberry, 3 times a day. The total ealing average (single or multiple) is less than Prayer of Healing and takes a higher level spell slot (4th level). The next bump up is at level 6 where Mass Cure Wound does a better overall healing but Conjure Woodland Being provides a better single target healing.
Cleric has access to the Prayer of Healing spell which provides a total overall healing capacity that only loses to Mass Heal. In comparison at level 8 (where Mass Heal is not a factor), Prayer of Healing heals a total of 246 over Mass Cure Wound 192 avg damage. As such, Clerics are effective at if the entire party is hurt and given enough time. However Prayer of Healing lies in between the boundaries of off combat healing and short rest. More so than the other fast casting spells.