League leaders Bayern start the weekend with a six-point cushion ahead of Sunday's game at Hertha, against whom Lewandowski has scored 10 goals in his last five games.
Lewandowski retained his FIFA's "The Best" male player title on Monday, two days after chalking up his 300th Bundesliga goal in Bayern's 4-0 thumping away to Cologne.
"There is no doubt Robert is the best striker the Bundesliga has seen in the modern era," Klinsmann, who won the 1990 World Cup with West Germany, told AFP in a video conference.
Klinsmann, 57, capped a glittering playing career, including spells at Tottenham, Inter Milan and Bayern, by captaining Germany to the Euro '96 title before coaching the German and USA national men's teams.
He admits being stunned when Lewandowski broke Gerd Mueller's record of most goals in a single German league campaign by netting 41 times last season. "In Germany, we never thought it would be possible to get close to that record."
With 23 league goals so far this season, Lewandowski remains on course to finish as the Bundesliga's top-scorer for the fifth year running.
"The way he continuously keeps that hunger, that drive to score and win titles is at a completely different level," added Klinsmann.
Hertha are winless in their last three games since a shock win over Dortmund before Christmas and are licking their wounds following Wednesday's 3-2 home derby defeat to neighbours Union in the German Cup.
Hertha have been a happy hunting ground for Lewandowski and Bayern recently.
The striker netted a hat-trick in a 5-0 win over Hertha in Munich last August and Lewandowski also scored all four goals in Bayern's 4-3 win in October 2020.
Klinsmann hopes Lewandowski "doesn't score too many" against his former club Hertha on Sunday, but "Bayern are clear favourites and need to keep their distance (in the table) over Dortmund, because they are not out of reach. Yet."
Pressure is mounting on Moenchengladbach coach Adi Huetter ahead of Saturday's home game against Union in the wake of Wednesday's humiliating 3-0 defeat at home to second tier Hanover 96.
Like Hertha, 'Gladbach are only four points from the bottom three in the table, while Union are up to fifth after last weekend's win over Hoffenheim.
A third straight defeat in all competitions would put Huetter in a precarious position and he is determined to "field the right eleven players who accept the situation and do something about it".
One to watch: Mahmoud Dahoud
Second-placed Dortmund hope their Syria-born German international recovers from a back injury to face Hoffenheim on Saturday having been badly missed in Tuesday's shock cup defeat at St Pauli.
Dahoud was outstanding in their previous 5-1 rout of Freiburg with a hand in three of Dortmund's first four goals before grabbing one himself.
"'Mo' has been in good shape, and of course we also missed his style of play," Dortmund coach Marco Rose admitted after Tuesday's cup defeat.
Dortmund could do with Dahoud's quick changes of pace, eye for space and ability to put in killer passes to compliment England's Jude Bellingham at Hoffenheim.
66 - the record number of consecutive Bundesliga matches in which Bayern has scored including last Saturday's win at Cologne.
20 - wins Bayern captain Manuel Neuer has enjoyed in 24 matches against Hertha Berlin with three draws and a single defeat.
15 - goals Lewandowski has scored in 20 matches against Hertha for Bayern and ex-club Dortmund.