Scorpion is a character based around defense and offense, he doesn't have the fastest walkspeed or run speed in the game, but he is very mobile once you get to pick his momentum.
He is the oportunity creator, and can hit or be anywhere in the screen when he really needs to.
I feel hellfire might be his best variation due the low risk he faces, as other scorpion players said before, you can deal with pretty much anything using his 3 variations across the whole cast.
Ninjutsu has a pretty good range, and was made to fight characters from midscreen, you can use these variation to fight off players who have bad experience with footsies or have bad footsies, in general this variation is half safer than inferno, but does much less damage compared to the rest.
Inferno has pretty good meterless damage, and some zoning tools that in work with his universal moves turns the tides so much and can bait a trade to punish by using his mobility. Its the high risk high reward variation, and the one everyone is having problems in how to deal with now, but the game is just 2 weeks old.
Hellfire is the most technical, has tools to fight fullscreen, up close and midscreen, and this variations is really setup based, something that we have an idea now, but still requires a lot to dominate, and he can do 42% meterless, which is about the same or more than inferno, but the execution requirement and accuracy to play this scorpion is something you won't see much across the board.