Use Sort Collection Action from Collection Manipulations VBO. Pass inputs of Collection Name, Field Name and for Ascending Order pass False.
This will get you a sorted collection in descending order.
Now to get the first value of the collection, you have to use loop stage and get the value in to a data item.
Loop Start
Use a Decision stage
For Yes, have a calc stage with expression, [CollectionName.Fieldname] and store in to a data item. Then ,link this to End
For No, Link to the Loop End.
You will get in to the loop just to read the value from collection and End the loop.