The best tank in 5e is probably Forge Cleric.
1. High defenses.
2. Very relevant resistances and immunities.
3. Spirit Guardians, making it difficult to slip past even if you wanted to, and forcing enemy aggro on yourself.
Granted, Tunnel Fighter Polearm Master Sentinel fighter or paladin or barbarian? They're arguably better, but rely on Unearthed Arcana.
Druids are kinda good at tanking by means of summons. Wall of Stench Kow, for example, repositions to entirely block the backline, has relevant damage and threatening opportunity attacks, but that's not really traditional 'tanking'.