Id say Wu-Tang is prob. the best of all time, but i really like The Roots a lot. Blackstar has some great songs but cant be considered one of the best ever. A Tribe called Quest has some really good songs but isnt in the top 3 for me.
Extremely tough call to make, but I'm going with Public Enemy here. Their lyrics have to be some of the most thought-prokoving words ever spoken on wax. Their style was also quite groundbreaking. "It Takes a Nation..." is still the most played rap record in my library and it's 20 years old. It was relevant then and it's still relevant today.