Most grammar books will tell you that you need "the" before a superlative like "highest" or "lowest", but this is not always true.
- We use "the" when there is a noun after the adjective e.g. the highest number, the highest proportion.
- When we put the noun before, we don't need "the" e.g. the number was highest, the proportion was highest.
So, compare these 2 sentences:
- The UK had the highest rate of unemployment.
- The unemployment rate was highest in the UK.