Just prioritize the boss's pet bug. I don't see this being a winning issue for you, so I would advise against fighting it unless the consequences are catastrophic. If it will take a year to fix this, during which time other critical flaws remain in the field, then yeah, I see your point. Keep fighting the issue a bit longer. Otherwise, just go with your boss's preferred priority.
Look at the big picture: you have a boss who is actually prioritizing bug fixes. Other people's bosses are telling them to add new features rather than fix bugs (just search questions here if you don't believe me), and they would love to have your problem.
Good workers do share their own opinion, but they also accept a final decision once it has been made. I do think it was ok to push back on the boss's stance initially, within reason. Within a healthy work environment, this sort of exchange is seen as a good thing. However, once the boss has made a decision, the best thing you can do is to accept it and get fully on board with the plan. This is true in all but the most exceptional circumstances.
I'd like to ignore this increased priority until other bugs are fixed...
You should definitely not secretly prioritize your work differently than your boss asked. That's just an all-around bad idea. While your concern for the application's success is admirable, this is the boss's call. Successfully working as part of a team means doing things you don't agree with from time to time.
If you think this is a systematic problem, propose a general solution. If bugs are being prioritized in an ad hoc way generally, with detrimental effect, you could try suggesting a systematic way of evaluating and prioritizing them (such as by number of users estimated to be affected, seriousness of the issue, and effort required to fix). Focus on the positive benefits of such a system, rather than any alleged problems in the way things were done in the past, and you may have a winning argument (If you already made a big deal about this request, though, you might want to wait a little while so it isn't fresh in your boss's memory).