This is the 14325th most frequent German word.
härtesten
Superlative form of "hart," meaning "hardest" or "toughest."
In this sentence, 'härtesten' is the superlative form of 'hart', meaning 'hard' or 'difficult', describing the noun 'Prüfung' which translates to 'exam'.
Das ist die härtesten Prüfung, die ich jemals bestanden habe.
This is the hardest exam I have ever passed.
Here, 'härtesten' modifies 'Spiel', meaning 'match' or 'game', to indicate it was the most challenging one.
Unser Team hat das härtesten Spiel der Saison gewonnen.
Our team won the hardest match of the season.
The word 'härtesten' describes 'Bedingungen', meaning 'conditions', signifying their high level of difficulty.