What is the name of Penny's most frequent ex-boyfriend in The Big Bang Theory?

Penny's most frequent ex-boyfriend in The Big Bang Theory is Zack Johnson.

Played by Brian Thomas Smith, Zack is Penny's recurring on-again, off-again boyfriend and appears more often than her other former partners, including Kurt, David, and Mike. He first appears in Season 3's "The Lunar Excitation" and repeatedly returns throughout the series.

Zack is written as Penny's friendly, attractive but exceptionally dim-witted former boyfriend, making him a comic contrast to Leonard and the scientists in Penny's social circle. Penny and Zack even had a drunken Las Vegas wedding, although neither understood that the marriage was legally valid. They later discover the truth and annul it.

A common mix-up is confusing Zack with Kurt, Penny's aggressive early-series ex-boyfriend who takes her television and owes her money. Another memorable detail is that Zack later becomes wealthy, marries Marissa, and asks Leonard to help father a child after learning he is infertile. Despite their history, Zack and Penny remain on friendly terms.