Guardiola swapped Bayern for Manchester City at the end of last season having brought a trio of successive league titles to the Allianz Arena.
Under the Spaniard's successor, Carlo Ancelotti, the Bavarians are three points behind surprise Bundesliga leaders RB Leipzig.
Rummenigge feels the style of football played by Bayern under Guardiola was the highlight of his time at the club and remembers it fondly.
"I would say that Bayern have never played better than under Pep," Rummenigge told Die Welt am Sonntag.
"We have always been successful, but rarely been described as beautiful.
"Under Pep, all of us suddenly raved. That's his legacy, I'd say."
Reports Philipp Lahm could be set to retire from football at the end of the season have intensified in recent weeks, and Rummenigge hinted at a future off-field role for the captain.
"We trust him," said Rummenigge. "This is a very good option.
"He is our captain and now more than just a footballer."