Phobos Lab is my favorite Ultimate Doom map, and the DRL version is such a well done creation. Also the UV and N! version with Nightmare Demons can make for some of the most heart pounding experiences in DRL but without getting exceedingly unfair.
Second is Mortuary for its theming, iconicness, its music, and of course its challenge, while having plenty of tricks available to mitigate its challenge. Limbo in comparison is a let down, but the greatly improved 0.9.9.9 version will make it a worthy counterpart to Mortuary.