I voted for Notorious BIG - Ready To Die, I hesitated between this one and NaS - Illmatic. I gotta say both are great albums, i'm not really into Eminem or Dre these times.
I always loved Action Bronson as a rapper, pretty similar vocally to Ghostface Killah, but both styles are completely different, not to be compared lol.
On the underground side of hip-hop I would maybe add a Jedi Mind Tricks album or some Necro for the death rap fans. French rap has lots of classics like IAM, Akhenaton, Shurikn etc, not to be forgotten.
If I had ONLY 1 album to choose let's say if i'm allowed to only have one rap or hip-hop album I would choose Legacy Of Blood by Jedi Mind Tricks it's an instant classic for me