plainest
plain-est
adjective
Superlative of 'plain'; most simple, unadorned, or unattractive in appearance.
"She chose the plainest fabric for the curtains."
Thesaurus
Synonyms: champaign, plain, field, unadorned, spare, unembellished, unornamented, bare, undecorated, unattractive, homely, unpatterned