Hip-Hop producers and their recordings

I see certain producers mentioned on a regular basis. Who are your favorite, or who are the best/most important, hip-hop producers, and what are their best/most important/most distinctive recordings? (I won't be contributing to this thread and am only marginally interested, but it seemed like a good idea. Might be useful to have one thread where lots of producers are discussed.) Comments about their different production styles are also welcome.