My favorites are UDAP with 3% major capsaicinoids (that's the important percentage to look for, MC. Also the scollville heat units or SHU are not nearly as important as it's based on a taste test by a panel... MC is much more telling).
UDAP with 1.34% (something like that number).
Sabre Red.
DefTec.
Not many other companies spend the money to test their product. They rely on marketing terms like SHU and the percentage given is often how much of the liquid in the can is the pepper but that has nothing to do with how strong it is.
Fox claims 4 million SHU, but that's not what's in the can. Most was only like .7% MC. Their newest, "most powerful" is only 1.4% MC. And the MC is how much of what makes pepper hot is actually in there. So not much stronger than anything else as most are around 1.2 or 1.3%.
It is a good option though.
"Strongest," "fastest," "most powerful," and other words like that are not direct comparisons so don't mean anything. They are empty advertising words. If they said stronger, then they'd have to back it up. As it is they are all "parity products." So they are considered to all be the strongest or fastest or whatever. In advertising the normal rules of English are out the window.
Sportco in Fife has the stronger version of UDAP and often Sportsman's warehouse does too. Outdoor Emporium in Seattle has the weaker UDAP. All normally carry Saber Red as well as Mace (R). I don't know if Mace (r) has it's products independently tested or has the device to test it themselves. But if they are even trying to tell the truth it's just as good as the others (excepting the 3% UDAP).
As you can tell I do like the UDAP a lot. They either have the test equipment or send their product out. They started by making bear spray, and bear spray is required by the EPA (it's considered a pesticide in the US) to have a certain MC content and be tested.