Most likely the most powerful(and most evil) robot every created by Dr. Robotnik is the nefarious Metal Sonic. Before Knuckles, before Shadow, before Silver, Fang, Jet, Blaze, or Johnny(though, arguably, not before Mighty), Metal Sonic was the very first, true "rival" character to Sonic the Hedgehog, and remains one of the toughest. Eggman's second attempt at a Sonic look-a-like robot remains by far to be his most successful model, able to match Sonic in both speed and strength. To do this, Metal Sonic employs a unique and complex AI system that lets him analyze and copy data from his foes through observation and battle, and then integrate that data into himself and even alter his internal structure. This "copycat" ability is what allows him to mimic Sonic and match his every move and mannerism, sometimes to the point where the robot believes he is the "real" Sonic. Despite this, Metal Sonic has yet to defeat his rival throughout their countless races and battles, and remains the most re-occurring badnik villain of the series. Metal debuted in Sonic CD, where he kidnapped Amy Rose and aided Eggman in his plan to seize the Time Stones. Since being narrowly defeated in said game by a wall, Metal made more playable appearances in side games like Sonic R and Sonic Drift, secret playable modes in the Adventure games, and a villainous role in Knuckles Chaotix where he transformed into a massive, red version of himself to combat Knux & company. He also appeared prominently in the Sonic the Hedgehog movie, where he (non-canonically) became Super Metal Sonic. He would not have another major appearance again until Sonic Heroes, where for the first time, he betrays his creator. Metal copies the power of Chaos(as well as all the other characters), thus gaining the ability to change his form, giving birth to both Metal Sonic Neo and the monstrous Metal Overlord. While his plans to annihilate Sonic and rule over robots everywhere failed, he was at some point recovered by Robotnik and apparently reprogrammed back into a mindless slavebot. In his most recent appearances(Sonic Rivals), he even has his own rival - Eggman Nega's Metal Sonic 3.0. Rivals for Rivals? C'mon.