I have to say monk. People forget that a level 20 monk has proficiency in all saves and get to reroll them in the off-chance they fail the first time. The invisibility keeps spellcasters from targeting you while also resisting most damage options if they end up landing. They also get evasion, which makes them even better against spellcasters.
As for melee combatants, if they don't get greedy, they can kite them by bonus action disengaging. Their movement is larger than most and if they're fighting a rogue, the rogue is going to have a hard time following up on their naturally good perception. Meaning it's a toss-up whether the rogue gets to even use it's sneak attack consistently.
Ranged combatants must deal with the monk's Deflect Missile which is basically a guarantee by level 20 unless something's strange is going on for a minimum of 26 damage from one attack. Once the Monk gets in their face, which is possible because of their BA dash and heightened movement, the Ranged combatant is at a serious disadvantage.
As for Druids, they aren't naturally proficient in CON saves and most animals they can wildshape into aren't either. Even with True Polymorph or Shapechange, they don't get Legendary resistance like most high Con creatures and 4 forced saves are hard to beat.
As for if they do become successfully banished or plane shifted, they can use Astral Projection to return to the fight.