The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel 1995 was awarded to Robert E. Lucas Jr. "for having developed and applied the hypothesis of rational expectations, and thereby having transformed macroeconomic analysis and deepened our understanding of economic policy" (from the official website of the Nobel Prize).However, somewhat surprisingly, I just noticed that Professor Lucas' most cited paper is NOT about rational expectations. According to Google Scholar (search result is here), his most cited paper is "On the mechanics of economic development" (Journal of Monetary Economics, 1988), which is cited more than three times as much as the second one, "Econometric policy evaluation: A critique." The citation of the former exceeds 13,000, which is amazingly high in the field of Economics (maybe in other fields, too).
The "mechanics" paper is a seminal pioneering work in endogenous growth theory and is built on the idea of Uzawa ("Optimum Technical Change in An Aggregative Model of Economic Growth", International Economic Review, 1965); because of this, the model is often called Uzawa-Lucas model. Professor Paolo Mattana, the author of "The Uzawa-Lucas Endogenous Growth Model" explain the model as follows:
R. Lucas, in the late 1980s, writes a path-breaking paper: by taking some initial intuitions of Uzawa (1965) a step further, he proposes a two-sector capital accumulation growth model where human capital plays the role of the key variable through which ongoing growth can be generated. Human capital is understood to refer, in Becker's tradition, to the skills and knowledge intensity of the labor force and is accumulated in the learning (or educational) sector via a linear constant-returns to scale technology, only requiring older vintages of human capital. The Uzawa-Lucas economy differs in a fundamental way from the standard neoclassical model; since a lower bound to the return of accumulation is implicitly imposed, the long-run growth rate basically reflects an endogenous equilibrium where only the "primitives" of a specific economy (endowments, technology and preferences) are relevant. Other factors, such as increasing population or exogenous technical progress, crucial in the traditional theory, have, conversely, no critical influence.