HAHAHHAHAHAHAHAHAHOnly the young and inexperienced Nikon shooters have a lens or two that are must-haves.
We old/experienced Nikon shooters will gladly make a list of 20 or more lenses that are "Must Haves", especially for Nikon Df.
I'm a young Nikon shooter and _I_ can give you an off the cuff list of at least 10 "Must Haves". Those will be the same as you'd hear BrianS tell you to buy. Then I'll tell you the OTHER 10 "Must Haves" as far as I am concerned and the odds are none of them will be on his list.
(Hint: No matter what ANYONE says, get a 105/2.5 Nikkor-P. Get one that was made Pre-AI and have it modified. You'll thank us all later.)
Canon made a good decision when going from FD to EF.
Nikon also made a good decision to stay with the F mount.
Take those two statements as you will