It depends on each person's particular skill level and feelings, really. I don't think that there is a specific question type that should be considered the most difficult in a general sense. I wouldn't, then, rely too much on others' input, since you will probably receive different answers.
The hardest questions are the ones that give you
the most difficulty. And in that sense, you should focus on those in order to improve this weakness and increase your GMAT score. It's better, though, to start with basic questions and build from there. Starting with 700-plus level questions would probably only cause confusion and discouragement.
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