Use the with superlatives (the best) and ordinals (the first) in most cases.
the + superlative/ordinal + noun
Avoid using a/an with superlatives (normally) because the reference is unique.
Is it the best/the first...?
💡 Common patterns: the best day, the most expensive option, the first time. You can sometimes omit the noun: This is the best (one).
✓This is the best restaurant in town.
✗This is a best restaurant in town.
Superlatives usually take 'the' because they refer to a unique extreme.
✓She was the first person to arrive.
✗She was a first person to arrive.
Ordinals normally take 'the'.
✓That was the most difficult part.
Most + adjective superlatives typically use 'the'.