Lowest-quality (5)
I believe the answer is:
worst
I believe this is a double definition.
'lowest' is the first definition.
(similar in meaning)
'quality' is the second definition.
(worst is a kind of quality)
(Other definitions for worst that I've seen before include "Best; least good" , "Beat" , "Lowest of the low" , "Of the poorest quality" , "Far from the best - defeat" .)