Wu are the only guys that are in the discussion in my mind. Bringing all those heavy weights into one group will NEVER be done again. They were a cultural movement. Protect ya neck is still .... just wow.
There are many great groups who have an album or two that you could put up against anybody, but what separates OutKast in my mind in their longevity, originality, and ability to make actual hits to go with masterpiece albums.
How many times you heard a single, bout the album, and wished you just bought the single?
With the true greats like OutKast and many of the artists you see in this thread, the best songs are usually not the singles.