First off, let me give my own definition of the word "legendary". To me, in this particular context, legendary would be someone who has had a profound impact in their region which transcends to other regions, possibly even globally, with respect to their genre of music which they perform. In this case, (gangsta)rap.
Granted, this legend doesn't necessarily have to be living, but even posthumously, his, her, or their contributions are well noted and celebrated in their particular community. Legends, as well as their contributions are TIMELESS. Years after they've dropped a debut album, or since their last effort, they are still being talked about in the rap/hip-hop community.
All that being said, my choice for the most legendary from the Midwest would be MC Breed.