The closest thing I've found to a magic pill after years of searching has been CBD. Smoking a high CBD cannabis strain is probably the fastest way to get it in your system in an emergency. If you're somewhere smoking isn't an option, they have tinctures you can put under your tongue. Unlike Xanax or other pharmaceutical solutions, CBD is natural, has mild to no side effects, isn't addictive, and is non-toxic i.e. it won't kill you if you overdose.
That said, the placebo effect can be strong.
it turns out you can still have extreme anxiety and a panic attack if the overt physical symptoms aren't there, however. anxiety is a state of mind as much as it is a set of physical symptoms.
In a case of cruel irony, I actually became quite depressed during the time I was taking propranolol. (Recent studies suggest this is not coincidence.) It also had no effect whatsoever on my baseline anxiety, which remained uncomfortably high, though it did prevent me from having the actual attacks that I considered my worst symptom.
At one point I stopped taking propranolol for a year. The highlight of this year was losing a prestigious international post-grad fellowship when I had a complete anxious meltdown during my final telephone interview. When it was over, I vomited. After that, I finally gave in and told a doctor to give me the antidepressants (in my case, 75mg venlafaxine).
For me, the effect was immediate, and I can only compare it to a case of cured tinnitus. All of a sudden, this uncomfortable thing that I could feel at all times and that I was always conscious of possibly overwhelming me was gone. Presto. I must say I immediately felt remorseful and a bit stupid for prolonging my suffering for as long as I did by refusing to try antidepressants.
I stayed on venlafaxine until a couple years after my wife and I started dating, at which point I felt like I could taper off, and I did. I'll leave it as an exercise to the reader to look up how much of a pain that can be.
So, my recommendation to fellow anxiety sufferers is to not be afraid to try antidepressants even though you may feel anxiety is the opposite of depression. At least don't settle for a half-cure until you've explored all your options.
Also there can be a strong placebo effect of taking the pill.
the medical standard of the 2000s - present would also be pushing SSRIs as the remedy for chronic anxiety, though they're quite useless for acute panic attacks.
third-line treatment might be barbiturates or similar, which would work in cases of acute panic. not sure if they are actually used however.