The answer will vary with perspectives and personal experience. From the answers posted it is obvious that “method” changes the difficulty factor.
From my experience I would say stalking antelope or Coues deer get my vote.
When I first hunted antelope I was determined to do it spot and stalk. It took 4 years but was an enjoyable challenge in some beautiful prarie country in 3 states.
As for the Coues, somewhere in a wonderfully remote canyon in Arizona there is a buck that I will slip up on and complete that goal in the near future.
When you think you are a good stalker, try stalking a coyote.