THE SUDDEN RESIGNATION of Assessor Mabel Teng gives Mayor Gavin Newsom the opportunity to give an office that for years has been rife with corruption and incompetence a complete makeover. It's badly needed: Although Teng was trying to get tough on downtown, there's still hundreds of millions of dollars in tax money the city isn't getting. The office is far too lax on fighting appeals by big property owners. And it's a moribund operation, technologically backward and badly in need of organizational help.

In fact, a good, active, progressive San Francisco assessor could take the lead on demanding changes in local tax policy and could become a statewide leader in the fight for changes to the old, unfair Proposition 13.

But the mayor's political advisors will almost certainly push to turn this appointment into another game of political musical chairs – and in the process, perhaps, free up a seat on the Board of Supervisors for one of Newsom's allies. There will also be pressure on the mayor to hold the job open until July 11, when an appointed candidate could avoid having to appear on the fall ballot. That would be a terrible mistake and a serious blow to the mayor's credibility as a politician who's broken with the worst traditions of the old Brown machine.

The next assessor shouldn't be chosen for political convenience. He or she has to be someone tough enough to stand up to downtown and the forces that are constantly pushing for more tax breaks. And he or she needs to have an understanding of the importance of public records and open government – the Assessor's Office is a font of vital public information, and right now much of it is virtually inaccessible.

The assessor doesn't have to be an expert in hands-on real estate appraisal, anymore than a good district attorney has to be a career prosecutor. It's a management job, and the top qualification ought to be the ability to take on a large, dysfunctional operation and turn it around. But unlike former assessors Doris Ward and Teng, the next assessor ought to have at least some relevant experience in the field and have shown some past interest in generating new revenue for the city. That's ultimately what this job is about: with the city's structural budget crisis forcing deep cutbacks in essential services, and no real end in sight, the city has to be looking aggressively at every possible source of revenue, and the Assessor's Office is at the top of the list.

Hundreds of the biggest corporations in town are lining up for property tax breaks, often making the flimsiest of arguments. Just as many others are almost certainly looking for (and exploiting) tiny loopholes the Assessor's Office doesn't even notice. A good assessor could save Newsom immense pain.

And beyond the politics and qualifications, the next assessor has to be – has to be – someone with unquestioned political integrity, someone who won't be accused of nepotism, campaign violations, or ethical sleaze. There are plenty of good choices – and most of them aren't at City Hall. This will be a legacy appointment for Newsom, and he needs to ignore the political hacks, move quickly, and make the right decision for the city.