lol @ Das Efx being on there. From those choices probably Mobb Deep on the strength of The Infamous and Hell on Earth alone. That said I'm probably overall a bigger fan of both UGK and 8Ball & MJG as I like more of their albums. I'm not gonna count Paul & J because they aren't really a duo unless you are counting just production wise or something.
Cold World Hustlers are probably my favorite rap duo ever though.
Honorable mention to Ghetto Mafia, Organized Konfusion, Madvillain, L.E.P. Bogus Boys, Group Home, Main Attrakionz, Heltah Skeltah, Smif-n-Wessun, Retro Sushi, Camp Lo, Rappin' Ron & Ant Diddley Dog, and uhhh some I'm sure I'm forgetting that I like more than most the duos on that list anyways. I fucks with the modern day Kris Kross Rae Sremmurd too, unlock the swag... the swag unlock.
Also how in the fuck is that list not going to have Gang Starr, Clipse, or M.O.P. on there but got BG Knoccout & Gangsta Dresta and Capone-N-Noreaga? lol