What is the superlative form of ‘simple’ -

Superlative adjectives are used to describe an object which is at the upper or lower limit of a quality (the tallest, the smallest, the fastest, the highest). They are used in sentences where a subject is compared to a group of objects.

Noun (subject) + verb + the + superlative adjective + noun (object).

The group that is being compared with can be omitted if it is clear from the context (final example below).

EXAMPLES
My house is the largest one in our neighborhood.
This is the smallest box I've ever seen.
Your dog ran the fastest of any dog in the race.
We all threw our rocks at the same time. My rock flew the highest. ('of all the rocks' is understood)
Source: EF