9900k's HT and possible 16 threads are kind of useless in 99% of all games. Even for streaming, you don't really need the extra 8 threads, especially at that price point.
If it's all about computing power in games, I'd say i7-9700k > i7-8086k > i7-8700k (due to binning).
Then again, CPUs aren't as important as GPUs in most games and if you are all about great UHD or WQHD+DSR graphics, you will run into any GPU's limit most of the time. At that point, you might as well get a 2600/2700(x) due to way better pricing without losing frames.