Is the proton the strongest acid?
No
Brønsted theory
In Brønsted theory $\ce{H+}$ isn't an acid at all. Acids lose protons, becoming conjugate bases, and $\ce{H+}$ is the proton itself.
Arrhenius theory
$\ce{H+}$ isn't an acid, because in this theory acids dissociate in water to form hydrogen ions.
Lewis theory
$\ce{H+}$ is an incredibly strong acid, but nuclei of other, heavier elements, for example alpha particles, are arguably stronger. I haven't found hard data for this and it may be rather difficult to get, these aren't your friendly neighbourhood Lewis acids ;)
Protonating agent
Bare $\ce{H+}$ might be the ultimate protonating agent. In proton transfer, with any Brønsted acid you could always try to find an acceptor weak enough that reaction constant would be lower then 1, proton would &quot;prefer&quot; to stay with acid then protonate base. That's not the case with bare proton, which is unbound. Therefore it may beat any Brønsted acid.
Why only &quot;might&quot;? Because whether bare proton can bind to a species depends on it's energy, which needs to be lower then proton affinity of a molecule to which it's supposed to bind. Otherwise proton may ionise the molecule instead, and even fuse with one of its nuclei, if energy is high enough.
Another thing is that acids like $\ce{H4O^2+}$, which are endothermic molecules, could beat a bare proton, because of their repulsive nature - they spontaneously lose protons and &quot;throw them away&quot; with positive charge of their conjugate base!