This is the 5416th most frequent Dutch word.
lelijkste
Ugliest.
In this sentence, 'lelijkste' is used as the superlative form of the adjective 'lelijk' (ugly). It describes the garden house belonging to the neighbor as being the ugliest among comparables.
Mijn buurvrouw heeft het lelijkste tuinhuis dat ik ooit gezien heb.
My neighbor has the ugliest garden house I have ever seen.
Here, 'lelijkste' is again in its superlative form, referring to the sweater in the store that is considered the ugliest among the selection.
Hij vond de lelijkste trui in de winkel en kocht die.
He found the ugliest sweater in the store and bought it.
In this case, 'lelijkste' serves to describe the painting being referred to as the ugliest among all the paintings the speaker has created.