Probably DUI since its the lesser of the evils.
Being caught with harder, gateway drugs suggests that this isn't a first time occurrance, and that there is most likely a history of drug use before arriving to the point of using one of the most illicit drugs out there.
Cheating suggests that there was an active thought process in arriving to the point of doing whatever it takes to get a good grade, which reflects extremely poorly on the student caught.
A DUI is terrible, sure, but seems to be chalked up to a stupid mistake, and seems less an evil than the other two. Some individuals could be barely above the legal limit, and be charged with DUI. Attending AA and going through community service can help improve a student's image. In fact, I think there was a thread earlier in which a student was arrested for DUI but subsequently gotten into a top 20 school (of course I wouldnt trust the forums for "true stories", but this seems more likely than getting into school with the other two .