Cubase 9 is a bit unique in that it stresses many different parts of a CPU. For the best performance, you'll want a processor that's very well rounded.
Depending on your budget, that means either an i7 in an LGA 1151 socket, or any of the 2011-3 socket processors. For professional use, I recommend the Xeon E5-1650 for a mid-range DAW, or for a high-end DAW, the E5-2687W.
If you're building a semi-professional DAW, the i7-6850k is a solid choice, as is the i7-7700k.