There are 7 main ways for consistently granting Advantage:
- Wolf Totem primal path of the Barbarian class
- Mastermind archetype of the Rogue class
- Shield Master feat
- Grappler feat
- Moon Druid
- Familiars
- Various spells
There are up and downsides for all of them, a lot depends on your DM and your party composition.
Wolf Totem Barbarian
Pro: The Advantage does not depend on the Initiative order, once you started the rage
You can consistently provide it for the whole encounter
Works for every enemy adjacent to you
Works for every ally
Con: Only melee attacks get Advantage
Limited number of uses per day, however with most DMs even 2 per day is usually enough
Mastermind Rogue
Pro: Unlimited number of uses
Benefits both melee and ranged
Con: One ally at a time
One enemy at a time
Expends your bonus action
Depends on the Initiative order, the enemy might be dead/out of reach before the turn of the ally you Helped
Shield Master
Pro: You can do it every turn
Every melee ally can attack the proned enemy
Con: Costs a feat
Expends your bonus action
Must use Shield
One enemy at a time
You must succeed on an Athletics contest. Not so hard after taking one level of Rogue, but you still should be Str primary
Works only for melee attacks, and even worse, ranged attacks get Disadvantage against prone targets
Does not work aginst Huge or larger creatures
Heavily depends on the Initiative order, the enemy might stand up before your allies have a chance to attack it
Grappler
Pro: You can maintain it without expending any action
The Advantage does not depend on the Initiative order, once you managed to restrain
Every ally can attack the restrained enemy
Con: Costs a feat
Must have a hand free
One enemy at a time
Expends one of your Attacks just to start a grapple, so best used after you have Extra Attack
Expends a whole action to impose restrained
You become restrained too
You must succeed on an Athletics contest. Not so hard after taking one level of Rogue, but you still should be Str primary
Does not work aginst Huge or larger creatures
Moon Duid: Wild Shape
Pro: Takes only bonus action to start
On 2nd level you have 6 hours of usage, assuming you get the standard 2 short rests
Lots of different forms, with Flight, Darkvision, Blindsense, etc
Huge amounts of extra HP, (Dire Wolf 37, equivalent to about 5 levels of Druid)
Can concentrate on spells, Flaming Sphere can improve your DPR significantly
Con: Can't cast spells
Usually horrible AC in Wild Shape, fixed by one level of Monk
Usually bad damage, fixed by 5 levels of Monk (Extra Attack)
Cant speak with party members
Good low level forms for control
- Dire Wolf (level 2, HP 37), proning on hit
- Giant Toad (level 4, HP 39), restrain on hit
- Giant Octopus (level 4, HP 52), restrain on hit, reach 15
- Giant Constricor (level 6, HP 60), restrain on hit, reach 10
Familiars
This is not listed below Spells, as you do not have to spend spell slots maintain concentraion. You might however need to multiclass or spend a ASI to gain the Find Familiar spell, so this is more like a feat.
While most familiars can't attack, they can use the Help action to provide Advantage on one attack per round.
Spells
Pro: Once you cast them, they usually do not take any kind of action to maintain
Con: Concentration can be broken
Spell saves are easier to succeed on than opposed Athletics checks (see Shield Master)
Number of spells extremely limited per day
Most of them take an action, unless you are a Sorcerer
Good low level spells to grant Advantage:
- Faerie Fire (one of the best, area, targets do not get to save every turn)
- Entangle (area, save costs an action)
- Maximillian's Earthen Grasp (if target gets out/dies, you can catch a new one)
- Hold Person (allies not only have Advantage, they Crit if they hit)
- Tasha's Hideous Laughter (you can target anything with an Int above 4)
- Blindness (does not use Concentration)
Best options for you
I would suggest you use the Shield Master way, as a Tank you should be using a shield anyway. Now you have a use for your bonus actions, and with 1 level of Rogue and a Rapier you do quite nice damage.
Best primary class is arguably Fighter, but a Bladelock works surprisingly well.
Anogher great option is Moon Druid with a few levels of Monk for Unarmored Defense, Extra Attack and Stunning Strike
It also works without the Monk part, but expect to be hit a lot, and your DPR is much less without Extra Attack.
The Sentinel feat can also add quite a lot of control/damage.
You have a lot more hit points than your party members, and Sentinel motivates the enemy to try and attack you, so those hit points are not wasted. It will also help you to stop those who try to get away.