It wasn't Tesla, though he was the first to engineer a complete ac
power distribution system including 3 phase.
I think it could be argued that it was Tesla. His original Big Idea
(while he was in the equivalent of High School!) was actually the
concept of a rotating magnetic field and what could be done with the
currents it would induce in nearby conductors.
Of course he didn't have the pure mathematical background of say
Steinmetz, but the two men were definitely not working in quite the
same arenas.
First I think it'd help if you specified which aspect of "AC" you
mean. What it covers today is the entire history of electrical and
magnetic phenomena from electrum and lodestones on, in a manner of
speaking.
Mark L. Fergerson