Exceptional and thoughtful responses all, to a most subjective question to be sure. I would add Wm. Thompson, Lord Kelvin. Not only did he advance EM studies and thermodynamics, and of course developed the Kelvin scale, he also developed an improved marine compass and (I believe) hydrographic equipment.

To go on a tangent, I would like to ask the assembled, "What quality(-ies) define 'greatness'?" How do we begin to define the "greatest" amongst a population, much less the greatest ever? I'd very much like to hear the criteria---full disclosure, this is a question I ask my philosophy students on a regular basis.