Cleric or Druid?

[GoblinGuy]

I need to play a tank capable of buffing and healing for an Underdark campaign. I need suggestions. Races and Subclass, along with the actual class itself, are welcome.

[Errata]

That's a lot of hats for one character to wear. Those are vague goals, and it's hard to know without more details what you mean by those things and therefore which builds would meet your criteria.

What sort of buffing did you have in mind? It's a limited role in 5e, because most things that might qualify as a buff are concentration effects, so you can only do one, and at the expense of something else you could be concentrating on. Plus if you're the tank, you'll be making regular checks to retain concentration. Some classes like paladin or bard may have extra buffs as class abilities, but for clerics and druids it's primarily their one concentration spell at a time.

And what degree of tanking did you have in mind? There are a lot of options for being able to absorb some damage, however there's not much reason for a smart enemy to want to attack the most defensive person in the party. Being a proper tank isn't just about being able to take a hit, but having some way to extend your influence over the battlefield so they may not have an alternative but to attack you. Feats can help with that, but it takes a lot of resources, and martial specialists are better suited to it. You'd be spreading yourself kind of thin to try to be a full tank and also accomplish multiple other roles and be a full spellcaster.

Both classes are capable of being healers. A cleric tends to be a bit more martially oriented, and has a decent selection of buffs, however they don't have a lot of crowd control options to fully keep enemies away from more vulnerable party members. Some of the domains might provide supplemental tools that improves their tanking however. A moon druid can tank reasonably well, but not while casting, and they tend to have poor AC and soak up the damage, meaning they may have trouble holding on to concentration effects even if they manage to cast one before wild shaping. And they can't re-up it without spending another of their limited wild shapes. They aren't a full time tank AND caster, just part time at both.

That having been said, I'll make a recommendation based on my interpretation of what you're asking:
Shepherd Druid, Hill Dwarf. 1 level dip in Life Cleric.

Tanking. Conjure Animals gives you a ton of meat shields to soak up damage and play interference to help with crowd control. It's one of druid's most powerful mid level abilities, and Shepherds are by far the best at it. Hill Dwarf makes you a lot more robust against damage than most other +1 wisdom races, and a darkvision race is an asset for an underdark campaign.

Healing. Druid has powerful healing options, made more powerful by the level 1 life cleric ability. This combo plus the Shepherd's unicorn aura makes one of the best healers in the game.
Buffs. In addition to regular Druid spell list, Shepherd has an area of effect buff ability as a bonus action that they can maintain without concentration, allowing you to do an occasional buff even while summoning.

[Citan]

I need to play a tank capable of buffing and healing for an Underdark campaign. I need suggestions. Races and Subclass, along with the actual class itself, are welcome.
If you want to tank "directly", then between Cleric and Druid I think the choice is obviously Cleric.

Then for Domain, Tempest, Life, Nature and Forge are all very interesting.

Life is obviously the best at healing, but REALLY best. Just the Channel Divinity will save party hide often.

Tempest is a mixed bag, but a good one: you'll have to invest a bit into feats if you really want to take advantage of your Domain features to tank, beyond that, it's a great mix of damage, battlefield control and tanking.

Nature is the most tanky of all: Plant Growth + Spirit Guardians + Sentinel/Shillelagh or Thorns Whip means you can keep enemies around you for a very long time, and you can also halve elemental damage for allies on reaction for free.

Forge is obviously the most resilient thanks to +1 magic armor and although I don't remember Domain benefits it should be good for taking hits and surviving.

In short:
Life = focus on heal/buff.
Tempest = versatility.
Nature = focus on tanking/aggro.
Forge = ??? (you'll have to decide for yourself).

[MaxWilson]

I need to play a tank capable of buffing and healing for an Underdark campaign. I need suggestions. Races and Subclass, along with the actual class itself, are welcome.

What stats did you roll?

[Nifft]

If you want to tank "directly", then between Cleric and Druid I think the choice is obviously Cleric.

Then for Domain, Tempest, Life, Nature and Forge are all very interesting.

Life is obviously the best at healing, but REALLY best. Just the Channel Divinity will save party hide often.

Tempest is a mixed bag, but a good one: you'll have to invest a bit into feats if you really want to take advantage of your Domain features to tank, beyond that, it's a great mix of damage, battlefield control and tanking.

Nature is the most tanky of all: Plant Growth + Spirit Guardians + Sentinel/Shillelagh or Thorns Whip means you can keep enemies around you for a very long time, and you can also halve elemental damage for allies on reaction for free.

Forge is obviously the most resilient thanks to +1 magic armor and although I don't remember Domain benefits it should be good for taking hits and surviving.

In short:
Life = focus on heal/buff.
Tempest = versatility.
Nature = focus on tanking/aggro.
Forge = ??? (you'll have to decide for yourself). I came here to say basically this, except probably not quite as well.

I endorse all this good stuff.

[FreddyNoNose]

I came here to say basically this, except probably not quite as well.

I endorse all this good stuff.
I will endorse your endorsing of this stuff.

[bc56]

Forge is obviously the most resilient thanks to +1 magic armor and although I don't remember Domain benefits it should be good for taking hits and surviving.

Forge = ??? (you'll have to decide for yourself).
At higher levels, forge gets bonus AC while in heavy armor, the ability to create nonmagical metal objects from raw materials (coins), resistance to fire, and, at very high levels, resistance to nonmagical BPS and immunity to fire.

[GoblinGuy]

What stats did you roll?
Point buy or standard array

[Cespenar]

You need the druid. In addition to filling all those roles, you can even scout around in Giant Spider form in the Underdark, which is a pretty big bonus, both for logistical and social reasons.

[Citan]

At higher levels, forge gets bonus AC while in heavy armor, the ability to create nonmagical metal objects from raw materials (coins), resistance to fire, and, at very high levels, resistance to nonmagical BPS and immunity to fire.
Thanks for bringing that into the thread. ;=)