The first impression is invariably that of Mrs. Bennet, because she controls most of the first and second chapters of the novel.
Austen introduces the Bennet family in the beginning of the novel merely by opening the door to the ongoing conversation that Mrs. Bennet had apparently brought to her husband prior upon the visit of a Mr. Bingley, who bought a country home near Longbourne.
As it is known, men at that age who come to property automatically seek a wife later. Hence, the most prominent first impression we get is that of Mrs. Bennet herself.
She is obviously annoying, talks too much about the same thing, is uncontrollably chatty, is ignored by her husband completely, seems desperate to marry off her daughters, and wants to hold the first claim of Mr. Bingley to her daughter, Jane.