For single-character, probably Ganyu.
The reason is that her charged shot lv2 attack can deal ridiculous amount of damage, especially if you’re aiming for the enemy’s weak point, which is always a guaranteed critical, and also stacking Critical DMG stats.
The new character Xiao can also deal big damage using his plunge attacks, although his Anemo elements kinda makes him lacking on getting damage from elemental reactions, unlike Ganyu, which can proc Melt from Pyro characters.
But TBH, most of *5 characters in Genshin can deal ridiculous amount of damage, as long as you:
- Properly equipped them (using the best weapons & artifacts, with the right stats & sub-stats).
- Leveling them up properly (both character & talents)
- Abusing the Elemental Reactions system, especially Melt and Vaporize (these 2 reactions can proc critical, which can deal bonus damage).
- Using buffs and debuffs, which you can get from various stuffs (foods, skills, equipments, elemental resonance, and reactions).