In some senses, gang crimes clearly pose the greatest challenge to law enforcement. Gang crimes are much more prevalent than either of the other two types of crime. This makes them a bigger challenge than the others. In addition, the root causes of gang crimes are very difficult for the police to do anything about. The police cannot do much to alleviate the socioeconomic problems that lead to the formation of gangs and to their penchant for criminal behavior.
On the other hand, if you define “challenge” differently, you can say that hate crimes are a bigger challenge. The reason for this is that they are more difficult to prove. To prove that a given crime was a hate crime, the law enforcement community (police and prosecutors) must prove that the crime was caused by hatred based on the victim’s group affiliation. In other words, they must prove that the victim was attacked because of their race or religion or whatever other characteristic and not for any other reason. This can be a greater challenge in a technical sense than proving a gang crime.
Gang crimes, then, are a bigger challenge to society and therefore to law enforcement. However, it may be hardest to prove someone guilty of a hate crime and get a conviction, thus making that (depending on our definition of “challenge”) a greater challenge to law enforcement.