Big list - without the main men
Oliver Heaviside : It was he who made Maxwell's equations work in the real world, he also did Laplace's work (also not mentioned yet) moving differential equations into ordinary algebra (the heaviside notation used 'p' instead of 's' & the unit object was a step rather than an impulse) but he got there and used it first.
What about Cavendish - he practically invented and discovered everything(!!) but was so paranoid & shy he wouldn't publish...
What about Babbage, really
inventing the whole of computing in the early 19th century - and of course Ada Lovelace, the first applications programmer.
What about Tommy Flowers, who built the first operational electronic computer, practically won ww2 with it & was forced by the british establishment to destroy it and burn his own blueprints while they gave it away to the yanks for nothing.... Not to mention Alan Turing, who may have been driven to suicide by that same establishment...
PS maxwell's was the first unification theory - 20th century physics in the 1900s