The best SSD for gaming, is any SSD.
NVME PCI M.2 drives aren't really going to offer you anything beyond what a standard SATA SSD offers, when it comes to gaming, EXCEPT perhaps slightly faster map, level or texture loading and even then it's probably not going to be a huge difference and is only going to BE a difference if the game files are actually stored ON the NVME or SSD drive.
If you have an SSD with the OS on it but your game files are stored on a secondary hard drive, you won't see any advantage at ALL from having an SSD. Now if you have the files stored ON the SSD along with the OS, or you have TWO solid state drives and one of them has your game files on it, then you will see some pickup in loading those files by you won't ever gain any FPS from a fast drive.
I'd recommend going with whatever size you feel you works best for you in one of the Samsung 850 or 860 EVO models, Crucial MX models or Sandisk Ultra II models. Even one of the less expensive SSDs is probably fine if you need a larger one and can find a good deal on it.
What is your budget for an SSD and what size are you looking to go with?