I have a piece examining that question at InSight. The short answer is maybe, but a lot of the reasons you might say that they are don't hold up all that well upon closer inspection. I didn't mention this in the article, but it's even more true if you take into account the influence over the American drug market, because the Zetas don't supply nearly as many cities as does Sinaloa.
That doesn't mean that the US and Mexico are wrong to go after the Zetas --maybe from the standpoint of starting with the low-hanging fruit, it makes a lot of sense-- but I don't think it's been sufficiently demonstrated that these guys are the worst of all. You have to start somewhere, of course, but as far as sending a message to the other gangs, it would be helpful if the governments would offer a better explanation as to why the Zetas are the worst, so as to disincentive those activities.