While Maxwell takes all the 'bounty points' in unifying the electromagnetic theory, personally I think the greatest contribution has to be of Heinrich Hertz, who in his very short life, actually proved the existence of electromagnetic waves. As Andrew mentioned during his presentation that although Hertz didn't think much of his own discovery, it led to all sorts of follow-up studies and experimentations. As a result of that, we all enjoy wireless communications today, which in my view wouldn't have been possible had Hertz not discovered the 'waves'.

Probably not the greatest, but high on the list for me..

Often forgotten is Jagadish Chandra Bose info here  who was proving things about microwave propagation and radio waves from the late 1890s.

As he did not patent anything, but did a large number of lectures and demonstrations, including in London, it is likely that his work and publications inspired those more commercial European minds to take up the development.

Oliver Heaviside - Maybe not remembered or appreciated for his contributions but much of his work is still applicable in various diverse fields today. Being self taught probably put him at a disadvantage with the establishment but valuable contributions to engineering and science in the 1800's were often by such individuals. On a practical note is his solution to transmission issues with long distance telegraphy (electromagnetic pulses along a waveguide in real terms). As is usual with the establishment who "know better," his solution using inductors to resolve the merging of pulses over distance along the telegraph wires (waveguide) wasn't adopted by the post office until 20 years later. This issue was comparable to the transmission of data at gigahertz rates in present day circuits.