BoxRec

What are the best examples of this phenomenon?

Not thinking about when one guy is decked a bunch of times, but when the action is back and forth, preferably with a high knockdowns-per-round ratio.

Archie Moore v Yvon Durelle - 8 knockdowns (4 each) in 11 rounds
Alex Stewart v Ezra Sellers - 7 knockdowns (4-3 in Stewart's favour) in 3 rounds
Kevin Lueshing v Chris Saunders - 7 knockdowns (5-2 Lueshing) in 3 rounds.
Naseem Hamed v Kevin Kelley - 6 knockdowns (3-3) in 4 rounds.
Ezra Sellers v Carl Thompson - 6 knockdowns (4-2 Sellers) in 4 rounds
Nigel Benn v Iran Barkley - 4 knockdowns (3-1 Benn) in 1 round.
George Foreman v Ron Lyle - 4 knockdowns (2-2) in 5 rounds
Michael Moorer v Bert Cooper - 4 knockdowns (2-2) in 5 rounds

Any more?

Lopez vs Endo 4 knockdowns (3:1 Lopez) In 2 rounds
Lopez vs Toyoshima 3 Knockdowns (2:1 Lopez) In 3 rounds
Lopez vs Olivares 4 knockdowns (3:1 Lopez) In 7 rounds

Danny Lopez really was the ultimate TV fighter

Also

Holt vs Torres 3 Knockdowns (2:1 Torres) In one minute

edit: almost forgot a famous one, Dempsey vs Firpo 11 knockdowns in 2 rounds (I think it was something like 8:3 Dempsey)

A more recent one:

Marcos Maidana vs. Victor Ortiz - 5 knockdowns (3:2 Ortiz) in 6 rounds

Patterson-Johansson III is a great example.

How about Patterson-McNeeley?

Various reports have McNeeley down 8, 11, or 13(!) times in 4 rounds.

I'm almost sure Floyd tasted canvas, too.

btw, Walcott was the ref.

in their World Bantamweight Championship match on 2nd December 1950 Vic Toweel knocked down Danny O'Sullivan 14 times, 8 of the knockdowns were reportedly scored in the fifth round, the fight was eventually stopped in the tenth round. Toweel asked "What must I do to keep him down?" This wasn't the only time O'Sullivan was knocked down a ridiculous amount of times on the 25th April 1950 Luis Perez Romero han knocked him down 11 times and eventually stopped him in the thirteenth round of their European Bantamweight match.