You know, the one that is so much fun to shoot yet really probably serves no purpose other than personal enjoyment.
I'm a bowhunter first and foremost and rarely gun hunt except with a flintlock muzzleloader but I do LIKE guns a lot.
My choice for "funnest" gun is pretty easy.
Stag Arms 2T, 5.56 nato chamber
w/ assorted preban goodies.

Soon to add either an Aimpoint M2 on a Larue mount or an Eotech 512 off the flattop.