The mitzvah of the four species is unique in that hiddur- a word which connotes enhancement, embellishment, and beautification - is an essential component of its fulfillment.

The verse states: This is my G‑d and I shall exalt Him (Exodus15:2). The Sages explained that the fulfillment of the term exalt is brought about by beautifying the performance of mitzvot. That is, any mitzvah which is performed for the sake of G‑d should be fulfilled in the most aesthetically attractive manner possible, as was mentioned earlier in connection with beautifying the sukkah. 

Although this is true of all mitzvot, the Torah emphasizes this point again for the mitzvah of the four species, for the verse states: the fruit of a beautiful tree. Our Sages explained that the term hadar - beautiful, majestic - used in the verse applies not only to the esrog but to each one of the four species.

These are the qualities that qualify an esrog as mehudar:

It should be completely clean of spots; it should have many protrusions and depressions on its surface so that it is not smooth like a lemon; the oketz [the pointed end where it was connected to the tree] should be depressed into the bottom; it should have a tower shape, i.e., broad at the bottom and narrowing towards the top; the shoshanta [the flower atop the pitom] should be whole on all sides and the pitom itself should be centered on the top of the esrog and parallel to the oketz.

These are the qualities that qualify a lulav as mehudar:

It should be green and show no evidence of dryness from top to bottom; it should be straight like a rod without twists or bends on any side; the top should not be even slightly broken or nipped; the central twin leaf and the ones adjacent should not be split but rather whole until their points; the leaves should not be separated from each other but should appear as a whole - separating only when shaken; the top point of the lulav should consist of a single twin leaf rising from the stem.

These are the qualities that qualify a hadas as mehudar:

The leaves should be completely green and show no evidence of dryness; the leaves on the stem should be grouped in rows of three, each sprouting from the same level of the stem; the rows of leaves on the stem should be close enough so that the tops of the leaves in one row extend to the beginning of the next row; the rows of three leaves should continue from the top of the stem for three handbreadths; the leaves should be neither excessively small nor large -approximately the size of a person's thumbnail.

The leaves should point up and cover the stem; the stems and leaves should be whole and not nipped at the top; the berries growing on the stem should not be more numerous than the leaves; twigs should not be growing from the stem between the leaves over a span of more than one and a half handbreadths; the length of the branch should be slightly more than three handbreadths.

These are the qualities that qualify an aravah as mehudar:

The stem should be red; the leaves should be narrow and long and the edges of the leaves should be smooth [not serrated]; the stem should not be nipped on top and the top leaf should be whole; the leaves should be green and show no evidence of dryness; the branch should have all of its leaves.

If one can obtain an aravah which grows at the edge of a river, this too is an aspect of hiddur.