First things first: Zack was a real sweetheart and a genuinely nice guy who just happened to be not very bright. He was so dim-witted that he actually thought Leonard and his gang were going to blow up the moon when conducting a lunar experiment. His stupidity actually repels Penny, who finds him insufferable.
Still, Zack proves time and again that he is a stand-up guy. He befriends the guys, actually dressing up as Superman to help them win the comic book store's New Year's Eve costume contest. In season 12, Zack, who is infertile, asks Leonard to act as a sperm donor so that he can father a smart child, confirming how much he admires the guys. Zack is definitely the dumbest, but he's also caring and a much better person than any of Penny's other boyfriends.
During a brief flashback in "The Staircase Implementation," the season three episode where Leonard reveals how he and Sheldon came to live together, Penny is shown in her bedroom as a teenager, waiting for the results of a recently-taken pregnancy test. Besides her is, presumably, her high school boyfriend, or a guy she at least slept with once. The test turns out to be negative and they both celebrate by high-fiving.
The character's not given a name and he doesn't even have any lines, but it's clear that he's not particularly sharp, at least at that point in his life.
TJ and Penny dated for six months. She reveals that she never knew what his initials stood for and that they never really talked, implying that they either had nothing in common or that there was simply nothing to talk about.
He is only mentioned once and it happens during Leonard's fantasy in the series' 100th episode, "The Recombination Hypothesis." It is therefore unclear if TJ actually existed, or if he was simply an invention of Leonard during his imaginary date with Penny.
The big guy who famously takes Leonard and Sheldon's pants in the pilot episode, Kurt was a real bully. He not only cheated on Penny, prompting her to dump him, he also teased and humiliated Leonard during Penny's Halloween party. During season 2, he also refused to repay Penny the money he owed her, finally doing it so that he could date her again.
A lout and an aggressive man, he is shown to also be quite stupid. He stops appearing after season 2, which was honestly great for fans because Penny was way too good for him.
After first breaking up with Kurt, Penny spends a weekend with a guy named Doug. She describes the affair as "36 hours of guilt-ridden sex," implying that she had no real feelings for him and basically just used him while still getting over Kurt.
It's easy to assume this is one of Penny's "dumb guys from the gym." Indeed, when Leonard meets him, he seems to have a very bad attitude, on par with Penny's type before she gets together with Leonard.
Fans of the show will remember Mike as the guy who posts intimate details of his relationship with Penny on his blog, causing her to violently break up with him. After Leonard gives her a pep talk that inadvertently convinces her to reconcile with him, she finds him with another woman, sending her into another hissy fit that eventually leads up to Leonard asking her out on their first date, which she accepts.
Mike hardly appears, but considering he was sleazy enough to brag about their sex life and skilled enough to convey it in his blog, he seems to be of at least average intelligence.
During his first appearance in the season 2 episode "The Hofstadter Isotope," Stuart is shown as the confident and artistically talented owner of the comic book store that bears little to no resemblance to the miserable and depressed man later in the series. Indeed, Sheldon describes him as "tall(er than Leonard), artistic and self-employed."
While Stuart is nowhere near as brilliant as Leonard or the guys, he is smart, being a college graduate and owning and managing his own business. He changes significantly over the series' run, but when he first dates Penny, he's definitely a bright guy.
A Mcarthur Grant recipient, David is a brilliant, handsome and successful physicist who first appears in the season 2 episode "The Bath Item Gift Hypothesis." Within the show's continuity, his work provided the first conclusive evidence of the existence of dark matter, effectively rendering the entirety of Leonard's work useless. Soon after meeting him, Penny starts dating him, much to Leonard's dismay.
However, intelligence does not a good guy make and it turns out David is married, which prompts Penny to dump him. Still, even Leonard admits David is smarter than him, going so far as to say that next to David, he looks like "one of those sign-language gorillas who knows how to ask for grapes."
