[Gothmog]

Interesting discussion... Though I would like to make one thing clear. We have to separate the two concepts of being "most powerful" and "highest in rank". One thing is clearly a reflection of a creatures characteristic and strength, whether it's physical or mental strenght, power of sorcery or something else. Rank doesn't necessary reflect the owner's power. The MoS could, theoreticaly (sp?) speaking, be a personal favourite of Sauron and therefore have had an easy climb up the ladder in the Barad-Dur ranks.

Now when I've covered that, I must add that I do not think these two are comparable. One has higher authority when it comes to war, leading troops, carry out the most important operations and such. The other has higher authority in his speciality; politics, administration and diplomatic missions. After all, people tend to be a bit suspicious to diplomats that comes riding on a giant monster, sends out a screams that freezes the blood in your veins and then torture you for a couple of hours...

For the most powerful, I'd go for the Witch King. For the one with higher rank in Barad-Dur, I'd say the Mouth of Sauron. For the one with higher rank in the military section of Mordor, I'd once again say WK, but I don't know who should be considered as the one with the highest rank overall.