Well, the 9900k is the fastest gaming CPU at the moment, but you are gonna pay a hefty premium for it. I would say that at 3440x1440 100hz you are gonna be GPU bottlenecked in 99% of games. I doubt you would feel a difference gaming compared to a x470 system with a 2700x, and that you can get significantly cheaper than the Intel bundle. Note that AMD showed a Zen2 chip at CES that beat a stock 9900k in CineBench with lower power consumption. Zen2 is gonna slot into x470 boards with a bios update.
If you are shopping in the 9900k bracket, consider that you can get Threadripper CPUs at similar and even lower price. If multi tasking, multi GPU and PCI lanes are important to you it's just something to keep in mind.
Once you have decided on a GPU its easier to recommend a CPU. the 9900k might make sense if your going for a 2080 ti as an example.