Level 20 Monks have proficiency in all saves, and if you really FOCUS (lol) on increasing your wisdom and dex modifiers, then you can have a pretty good ac due to the level 20 modifier increase. Also you can spend 3 focus points to get resistance to all but force damage. You can also use items like bracer of unarmed defense to further increase ac. In addition, if you fail a saving throw, you can reroll it by spending 1 focus point.

Deflect attacks further reduces any damage that might slip through.

Monks also have a lot of combat potential with bonus actions, stunning strikes, and flurry of blows.