I would honestly say knife before KitchenAid.
As much as I love my KA, a knife is something you usually use daily. The Chef's was actually our first as it's so versatile.
I personally have a thing for nice (wooden!) cutting boards in different sizes and made from different woods but I think that's already in the "obsession" area.
And, as simple as it sounds, a set of simple metal bowls in different sizes. I can't really explain it but they somehow make everything you do feel so much more professional. We have those https://www.amazon.de/dp/B00ZVUZVFI/ref=cm_sw_r_cp_apa_i_gYROFb6J3YC7X