From “To the Virgins, to Make Much of Time” by Robert Herrick

That age is best which is the first,
When youth and blood are warmer;
But being spent, the worse, and worst
Times still succeed the former.

Which word in the passage from “To the Virgins, to Make Much of Time” by Robert Herrick is an irregular superlative adjective?

The word "worst" in the passage is an irregular superlative adjective.