If this is to be displayed in the page header, your only option is a textbox; you can't add tablix type parts to page headers/footers.
The textbox expression would be something like:
=First(Fields!MyValue.Value, "DataSet1")
By using an aggregate expression like this you can make sure only one value is returned (even though you might always have only one) and you can also specify the aggregate's Scope; in this case the DataSet you want to query.
If this was going in the report body I would still recommend the same approach, though I wouldn't go so far as to call it best practise, any would work so it's really personal taste.
However, if you had multiple fields returned by the SP but still only one row, in that case I would recommend a table style tablix with one header-level row; easiest to maintain and layout.