I think the strongest Genshin Impact character is Zhongli. While it's a hard question to answer given that different characters have different roles, the five-star Geo Archon provides so much value in terms of his shields, and he's incredibly simple to build.
All you need to do is get him as much HP as possible, then have him whack down a Geo pillar to give your party almost unbreakable defense. When it disappears, simply use the skill again and you'll almost never take damage. He can also pop in and occasionally drop a meteor that decimates shields and turns enemies to stone.