What is the strongest acid?
The problem with this question is that the exact answer is "it depends..."

First off, it depends on your definition of acidity and how you measure it. Everyone seems to be using Brønstead acids (i.e. $\ce{H+}$ donors). I see two different measures in other answers:


Proton affinity: This is a gas-phase measurement of $\ce{A^{−} + H^{+} -&gt; HA}$
Hammett acidity ($H_0$): This is a solution measurement, given by $\mathrm pK_{\ce{BH^+}} - \log\left(\frac{[\ce{BH^+}]}{[\ce{B}]}\right)$


Secondly, it depends on medium as mentioned by LDC3.


Leveling effect: The solvent leveling effect reflects the lowest possible $\mathrm pK_\mathrm a$ in a particular solvent, based on the basicity of the conjugate base. So you need $\ce{HF}$ or fluorosulfuric acid to reach low $\mathrm pK_\mathrm a$.


So the problem in my mind is that carboranes and fluoroantimonic acid are solution measurements, but $\ce{HeH+}$ is a gas-phase measurement. It does have the highest gas-phase proton affinity. But I'd put my money on things I can use in lab.

Incidentally, the Reed group prepared the fluorinated carborane acid this year: Angew. Chem. Int. Ed. 2014, 53 (4), 1131–1134. So that compound, $\ce{H(CHB11F11)}$ wins the crown for strongest solution-phase Brønstead acid (for the moment, at least).

There's also a nice review article "Myths about the Proton. The Nature of H+ in Condensed Media" by the same group in Acc. Chem. Res. 2013, 46 (11), 2567–2575.