Sans

Sans has killed many. He has been through everything. He is the most powerful. He is.... not fighting anything anymore. Yep, Sans is now a pacifist. He stopped trying a while back.

Wah War Veteran
Sans was a general in the Wah War. That was his last fight. The other general was Waluigi. Needless to say, Sans' side won the war.

He once fused with Thanos unintentionally, forming Sansos, but that isn't important.