This is the 16770th most frequent Russian word.
сложнейшие
"Most complex" or "most difficult."
The word 'сложнейшие' is used in its superlative form to describe the theories as exceptionally complex.
Я изучаю сложнейшие математические теории.
I study the most complex mathematical theories.
The word 'сложнейшие' is used to emphasize the severity and difficulty of the challenges presented.
Он преодолел сложнейшие испытания в своей жизни.
He overcame the most difficult challenges in his life.
The word 'сложнейшие' is used to classify the task within a group of exceptionally difficult ones.