Question: What is the most powerful acid?

Fluoroantimonic acid

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Fluoroantimonic acid is an inorganic compound with the chemical formula H
2FSbF
6 (also written H
2F[SbF
6], 2HF·SbF5, or simply HF-SbF5).  It is an extremely strong acid, easily qualifying as a superacid.  The Hammett acidity function, H0, has been measured for different ratios of HF:SbF5.   While the H0 of pure HF is −15, addition of just 1mol % of SbF5 lowers it to around −20.  However, further addition of SbF5 results in rapidly diminishing returns, with the H0 reaching −21 at 10mol %.  The use of an extremely weak base as indicator shows that the lowest attainable H0, even with > 50mol % SbF5, is somewhere between −21 and −23.[1][2][3]
The "canonical" composition of fluoroantimonic acid is prepared by treating hydrogen fluoride (HF) with antimony pentafluoride (SbF5) in a  stoichiometric ratio of 2:1. It is the strongest superacid based on measured H0 value.  Only the carborane acids, whose H0 could not be directly determined due to their high melting points, may be stronger acids than fluoroantimonic acid.[4] It has been shown to protonate even hydrocarbons to afford pentacoordinate carbocations (carbonium ions).[5][6]
The reaction to produce fluoroantimonic acid results in formation of the fluoronium ion:
SbF5 + 2HF → SbF−
6 + H2F+
The acid is often said to contain "naked protons", but the "free" protons are, in fact, always bonded to hydrogen fluoride molecules.[7] It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity.  The protons easily migrate through the solution, moving from H2F+ to HF, when present, by the Grotthuss mechanism:
H2F+ + HF ⇌ HF + H2F+
Fluoroantimonic acid thermally decomposes at higher temperatures, generating free hydrogen fluoride gas.  It is exceptionally corrosive and can only be stored in containers lined with Teflon.
Structure
Two related products have been crystallized from HF-SbF5 mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas [H
2F+
][Sb
2F−
11] and [H
3F+
2][Sb
2F−
11]. In both salts, the anion is Sb
2F−
11.[8] As mentioned above, SbF−
6 is weakly basic; the larger anion Sb
2F−
11 is expected to be still weaker.
The following values show that fluoroantimonic acid is much stronger than other superacids[4] based upon the Hammett acidity function. Increased acidity is indicated by smaller (in this case, more negative) values of H0.
Fluoroantimonic acid (-23 < H0 < -21)
Magic acid (H0  = −19.2)
Carborane acid (H0  < −18)
Fluorosulfuric acid (H0  = −15.1)
Triflic acid (H0  = −14.9)
Applications

This extraordinarily strong acid protonates nearly all organic compounds. In 1967, Bickel and Hogeveen showed that 2HF·SbF5 will remove H2 from isobutane and methane from neopentane to form carbenium ions:[9][10]
(CH3)3CH + H+ → (CH3)3C+ + H2
(CH3)4C + H+ → (CH3)3C+ + CH4
It is also used in the manufacture of tetraxenon gold compounds.
Materials compatible with fluoroantimonic acid as a solvent include SO2ClF, and sulfur dioxide; some chlorofluorocarbons have also been used. Containers for HF-SbF5 are made of PTFE.
Safety
HF-SbF5 is extremely corrosive, toxic, and moisture sensitive.[6] Like most strong acids, fluoroantimonic acid can react violently with water, owing to the exothermic hydration. Consequently, it cannot be used in aqueous solution, only in hydrogen fluoride as solvent.
Fluoroantimonic acid cannot be stored in glass, as it will dissolve it. It must be stored in a PTFE (polytetrafluoroethylene) container.[11]
See also
Fluoroboric acid
Fluorosulfuric acid
Hexafluorophosphoric acid