based on my experience with susie, i can say without the shadow of a doubt that she is one

That is the best choice when the truth of the clause following doubt is assumed, as in negative sentences and questions.

Note that, in certain kinds of sentences, the choice of conjunction can carry subtle differences in implication. Some 86 percent of the Panel prefer that in the sentence I doubt it will rain tomorrow (where the expectation is that it probably won't rain), with whether getting the preference of only 6 percent and if getting 7 percent. When the expectation for the outcome is negative, that tends to be used. Only 6 percent said they would favor if in this sentence, probably because if has a more informal tone. In our 2008 survey, 51 percent indicated that they would use that, while 43 percent preferred whether in the following sentence: At one time it was doubtful the company could recover from its financial difficulties, but the government loan seems to have helped. When doubt and doubtful indicate strong uncertainty, the Usage Panel prefers whether and that over if.

Usage Note: The choice of what conjunction to use following doubt and doubtful is a perennial usage problem.

Based on my experience with susie, i can say without the shadow of a doubt that she is one