adjective
DUBIOUS : It's one of the most unique pieces of architecture in Spain.
GOOD : It's one of the most unusual pieces of architecture in Spain.
◆◆◆
Unique usually describes something that is the only one of its kind: 'Lennon will be remembered for his songs and for his unique sense of humour.'
In informal styles unique is sometimes used with more/most/rather etc. to mean 'unusual': 'This rather unique little restaurant is very popular with the locals.' This use of unique is considered by careful users to be incorrect.