What is your budget?
GTX1080ti prices are all over the place.
$$750 might be typical.
One rule of thumb is to budget 2x the cost of the cpu for the graphics card.
That would indicate about $350 for your CPU.
Today, the best recommendation I would make is for a I5-8600K and plan on overclocking it.
Tomorrow, the i5-9600K will be a slightly better buy.
Many are buying the i7-8700K for the 12 threads it provides, and the ryzen 2600 series with 12 threads or even the 2700 series with 16 threads.
But, for just gaming, few games are going to use more than 4 threads effectively.
Clock rate means all. Intel K chips can oc near 5.0, but ryzen tops out at 4.2 and the ryzen ips is slightly worse.
If your games are fast action shooters, graphics is most important and you might get away with a I5-8400.