What a fun question that truly gets the reader/audience to think about the different aspects of these two characters! Let's take the two of them separately:
For Duncan a few important adjectives are: trusting, loyal, giving, and oblivious. More importantly, though, is WHY I would choose those adjectives. Well, Duncan is a good king, ... as much as an oblivious king CAN be good, anyway. The first three adjectives indicate that he is a good king. He isn't worried about the proximity to Macbeth even while evil is swirling around violently in his subject's mind. Duncan is loyal because he is rewarding his faithful subjects for their valor in battle at the beginning of the play. This is the same reason why he is giving. Unfortunately for him, though, Duncan is also oblivous. He has absolutely NO idea that there is a possible plot against him, nor does he realize that such a plot can and does exist. Due to his lack of security (and/or lack of judgement as a result), poor Duncan is murdered in his sleep.
For the witches a few descriptive adjectives would be as follows: ugly, cunning, evil, and correct. They are truly ugly. Even Banquo says "What are these / So withered, and so wild in their attire?" It's never pretty when a woman has a beard. Now, let's take the two words cunning and evil. Here we get to the root of the witches and their predicitons: do the witches simply predict the future or do they plant the seed in Macbeth's mind to grow the germ of evil? These two adjectives deal with that very concept. This decision is up to you. Are they simply cunning in their predictions and in their boldness in telling Macbeth? Or are they truly evil (which the reference even to them "killing swine" would indicate) and suggest the murder of Duncan to Macbeth? (I am of the latter persuasion.) Further, though, we have to admit that they are also correct. Just look at the predictions! They tell Macbeth to "Beware Macduff!" that "none of woman born / Shall harm Macbeth," and that "Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him." All of these things do, in fact, come true, ... no matter what your persuasion as to why.