Use only parent terms in view

If you want to display the top-level terms of a given vocabulary, create a "term" view ("View type: Term") with these basic settings: a field for "Taxonomy:Term", an argument for "Taxonomy: Vocabulary ID"

Then to filter:
1) add a relationship to "Taxonomy: Parent term"
2) add a filter on "Taxonomy: Term ID", using relationship "Parent", with operator set to "Is empty (NULL)"

This will list only top-level terms for the given vocabulary.