The name of Dota 2's most powerful neutral creep boss is Roshan.
Roshan is a unique neutral boss found on the game map. Unlike ordinary lane creeps or jungle creeps, he is hostile to both teams and offers major rewards when defeated. Either team can attack him, so contesting Roshan is a common source of decisive battles.
His signature reward is the Aegis of the Immortal, which gives its holder a one-time resurrection after death. Roshan reappears after roughly ten minutes and becomes more difficult to defeat as the match progresses. Players often say “Rosh” as shorthand for Roshan.
Roshan should not be confused with Tormentor, another powerful neutral objective introduced later. Tormentor is also important, but Roshan is the canonical answer to the question asking for Dota 2's most powerful neutral creep boss.