1. Photoelectric Effect (for which he won the Nobel Prize).
2. Brownian motion of particles suspended in a fluid.
3. Special relativity, including effects of time dilation, length contraction, etc
4. Equivalence of mass and energy (E = mc^2)
5. Equivalence of gravitation and acceleration.
6. General Relativity
7. The initiation of Quantum Mechanics, which he never fully accepted as it developed.
ADDED: Of course, Dr Einstein built upon the work of others, including Pythagoras, Newton, Maxwell, Faraday, Boltzmann, Planck, Curie, Lorentz, and about a hundred others. So did all the folks listed. So do all scientists. In 1905, the academic exercise of trying to list them all was not as essential to publication as it seems to be today. None of these folks ever described the equivalence of mass and energy, the equivalence of gravity and acceleration, or the effects of gravity on light like Einstein did. They fiddled around at the edges, but they never had the guts to put it all together and publish it. Newton sat on some of his results for 30 years, then rushed to publish when he feared that Leibniz would steal his thunder. At least Einstein wrote down what he was thinking, and shared it with us.
He tried to steal the credit for work originally done by…
Jules Henri PoincarÃ©
Hendrik Antoon Lorentz
George Francis FitzGerald
And perhaps others. Physicist Max Born was a friend of Einstein’s, but he had harsh words to say about his habit of plagiarizing the work of other theoreticians.
“[Einstein’s] paper ‘Zur Elektrodynamik bewegter Koerper’ in Annalen der Physik. . . contains not a single reference to previous literature. It gives you the impression of quite a new venture. But that is, of course, as I have tried to explain, not true.” –Max Born (1956)