Question

Select the word or words that would correctly complete the following sentence. That is the _____ box of pens I could find. expensivest least expensive less expensive least expensivest

Answer 1

That is the least expensive box of pens I could find. Expensivest is not a real word.

Answer 2

That is the least expensive box of pens I could find. 
There is no such word like expensivest.  
On the other hand, using less expensive in this sentence doesn't make any sense.