Quark
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964. Quarks were introduced as parts of an ordering scheme for hadrons, and there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Accelerator experiments have provided evidence for all six flavors. The top quark, first observed at Fermilab in 1995, was the last to be discovered.
George Zweig
As pointed out by astrophysicist John Gribbin, Gell-Mann deservedly received the Nobel Prize for physics in 1969, for his overall contributions and discoveries concerning the classification of elementary particles and their interactions; at that time, quark theory had not become fully accepted, and was not specifically mentioned in the official citation of the prize. In later years, when quark theory became established as the standard model of particle physics, the Nobel committee presumably felt they couldn't recognize Zweig as the scientist who first spelled out the theory's implications in detail and suggested that they might be real, without including Gell-Mann again.
Nevertheless, in 1977 Richard Feynman nominated both Gell-Mann and Zweig for the Nobel prize, presumed to be his only
nomination for such.
Whatever the reason, despite Zweig's contributions to a theory central to modern physics, he has not yet been awarded a Nobel prize.