Zach pushed the girl to a drug addiction, so I'd love to hear that argument.
Are you actually familiar with the show, or are you using one of these Zack Morris is Trash episodes as your sole source of that information? The reason I ask is because Jessie has a history of being overly hard on herself throughout the series, especially with regard to her education. She begins that particular episode in an already more exaggerated state of her usual self in that capacity.
I won't deny that Zack is a complete pile of crap, but that's his character, and I always got the vibe that you're supposed to think more of Jessie because she's supposed to be more level-headed than any of the other main characters. It's hard to do that when you take away the sort of motivations that Zack has for his actions and replace them with the sort of directionless spite that drives Jessie's actions. Jessie's straight-up shitty toward her friends on a regular basis throughout the series, but without the comedic scheming that Zack employs. Furthermore, I don't recall Jessie mistreating people solely to get ahead as Zack does; she has nothing to gain from treating them badly, yet continually does so. Plus, it's not as though Zack's schemes are always funny, but they're what drive the show; they're expected.
Honestly, of the six main characters, I'd say that Lisa's the least terrible, but even she has her moments with regard to her treatment of Screech. (That said, Screech is a creep, so much of that is easily forgiven.) She at least doesn't have the sort of repeat offenses that the other characters tend to. If we're counting the first and last seasons, though, I'd say that Tori's probably the least terrible main character. All of the characters have their issues, though, as I mentioned before.