The Reds boss is the only manager with a winning head-to-head record over the Spanish coach and the Manchester City man believes the challenge posed by his side in the past two seasons has been the most difficult he has faced.
Pep told DAZN: "The toughest opponent I've ever faced in my career is this Liverpool from last year and this one.
"If you let yourself be dominated, they lock you in the area and you can't escape. When you dominate them, they run into space like no one else. They are very fast going backwards.
"They are very strong strategically. Their players have great mental strength. [Klopp] is the rival who has made me think about how to beat him the most."
Guardiola went up against Jose Mourinho's Real Madrid during his spell as Barcelona manager, although he was not as daunted by them as much as Liverpool.
He explained: "I have always said that Real Madrid are a very strong team in my career, they helped me to become a better coach with tough matches and competitions with Jose Mourinho, [Manuel] Pellegrini and all the coaches they had.
"If you ask me which has been the most difficult rival for me to face, it has been Liverpool. The first years when I arrived in England, Liverpool were a bit weaker than now and Real Madrid were stronger. Now, this Liverpool are the hardest I have come across in my coaching career."