Well, I loaded a dictionary file into 'Microsoft Access' and wrote quite a bit of scripting to go with it, so I have some good suggestions.
It's best to try shorter words, rather than longer ones, because there are more letters that aren't present, and hence more chance of them guessing wrongly.
To foil people asking the vowels (A, E, I, O, U), here are some choice words:
If your opponent read thbz's node on Letter frequency in several languages, the following words don't contain common letters such as E, T, A, O, N, I, H, S, R, D and L:
And if those words can't vanquish your opponents, I don't know what will.