The giant Autobot city that transforms into a robot is Metroplex.
Metroplex is one of the largest Generation 1 Transformers characters. In city mode, he functions as a fortified Autobot headquarters, complete with ramps, towers, repair facilities, and launch areas for smaller Transformers. He can then convert into a colossal humanoid robot for combat, making him both a base and a warrior.
The character was introduced during the Transformers toy line’s expansion around 1986, when increasingly large “city-bot” figures became a major part of the franchise. Metroplex’s Decepticon counterpart is Trypticon, another transforming city-sized character. That pairing can cause confusion, especially because both are associated with enormous fortress and city modes.
Metroplex is not the same character as Fortress Maximus, Omega Supreme, or Cybertron. Those characters also have huge scale or important base-related roles, but Metroplex is specifically the Autobot city-former described in this question.