Guardiola discussed the Dane after Saturday's narrow 2-1 win over the Bees.
"They are an extraordinary team," said the City boss. "What they do always makes sense.
"Every corner is a headache. They are compact. This season, they have a high press. Thomas is one of the best."
On Frank eventually moving to a bigger club, Guardiola replied: "It is just a question of time.
"I'm good in a few things, one of which is reading when the manager is good. It is going to happen."
In response, Frank said: "First, thank you for that, Pep - it is always nice.
"There are a lot of reasons why I admire Pep. He always shows class and did it when we beat them here last time, almost 50 games ago.
"I am very happy at Brentford but maybe one day I will try something new."