As a rule, rhododendrons are not fragrant but there are some notable exceptions.
Fragrance in rhododendrons is genetically linked to paler colours normally whites through to pale pinks.
The best known are the R. edgeworthii Hybrids like R. Fragrantissimum also some of the R. maddenia subsection and their hybrids. Larger flowers are normally from the R. fortunei parentage like R. Loderi. There are a few scented tropical forms called Vireya rhododendrons.
Also some have aromatic foliage which should not be forgotten. The best known is R. cinnabarinum with its silver blue foliage with cinnamon-like aroma when rubbed against or crushed. It passes this on to its children, e.g. R. Alison Johnstone.
The deciduous azalea ponticum (Rhododendron luteum) can on sunny days with one single shrub envelope an entire garden in a sweet, heady cloud of fragrance.