Right now, the only chips based on the six-core die used in the Core i7-8700K are the series of Coffee Lake desktop CPUs to which it belongs. If you want the absolute top single-threaded performance, it's gotta be a Coffee Lake CPU. For professional users that don't necessarily need the assurance of ECC memory, a Coffee Lake-based machine like Boxx's Apexx S3 could provide the maximum performance on the market right now.
The Apexx S3 series is certainly unusual for a workstation. Not only does it make use of a standard desktop CPU rather than a Xeon, but Boxx even throws a modest overclock on the chip. The company guarantees that the S3's CPU will run at 4.8 GHz on all six of its cores even under sustained workloads. That's going to make a lot of heat, but the Apexx S3 comes equipped with a custom-designed chassis and an integrated water-cooling system that should keep things cool.
The standard configuration of the Apexx S3 includes 16GB of 2666 MT/s DDR4 memory, a Quadro P2000 graphics card, and a 512GB PCIe SSD. Buyers so inclined can outfit the workstation with up to 64GB of memory, three M.2 SSDs, and eight SATA drives. The Apexx S3 can also take a trio of graphics cards from Nvidia's Quadro or Tesla families, or AMD's Radeon Pro series. The power supply in the machine apparently maxes out at 650 W, though, something that might limit multi-GPU configurations.
Boxx is going to be showing the Apexx S3 at Autodesk University in Las Vegas starting November 14, so folks headed to that event can check out the new machines there. However, the company is already selling them on its website. The base configuration listed above starts at $3630.