The propellants in KSP: Liquid Fuel, Oxidizer. (Sorry Monopropellant, you are for another discussion). The classics, designed to be unambiguous.
However, the KSP community has always tried to put a definition on them. Liquid Fuel was easy - liquid hydrogen. It can run as a high efficiency vacuum engine, or as a jet engine along sea level. As a first stage or in a NERVA. Liquid Hydrogen checked all boxes.
Oxidizer, on the other hand, has stayed in debate if not given up on. Liquid Oxygen feels like a good candidate, but fails in one area: hypergolic-ness (Okay, that’s the dumbest way to say that).
What I mean is that to keep with KSP’s engine style of instant ignition and many starts and stops, the propellants probably have to hypergolic. (If you aren’t a rocket nerd, that means they ignite instantly upon contact). Unfortunately Oxidizers that are hypergolic are quite rare, and almost all of them only are with fuels like hydrazine.
But then I found one: (really a family of propellants, but I settled on one.) fluorine or one of its chemical siblings, chlorine trifluoride. These are hypergolic with pretty much anything (It doesn’t care if it’s liquid hydrogen or flippin concrete!) Also, with proper treatment, ClF3 can be stored in tanks made of many different metals. It even has a high ISP!
Why isn’t this miracle propellant being used? Well it turns out that government officials don’t like exhausts of hydrochloric acid and hydrofluoric acid in the air. Kerbals? They love going to space more than anything! Who cares? Even in the original KSP, there is a green goo on the launchpad. Guess what is a yellowish-green? ClF*.
All in all, a real propellant that has the magical aspects of oxidizer: Instant ignition and good efficiency.
flamma est ubi?
*Of course it isn’t ClF3! The concrete would be on fire!