PLEASE SEE UPDATE, BELOW
Ben Morison, a leading young scholar of ancient philosophy at Oxford University, has accepted a tenured appointment at Princeton University, starting next year. Morison will join Hendrik Lorenz (recently tenured), Alexander Nehamas, John Cooper (who is reported to be thinking of retiring), and Christian Wildberg (in Classics) to sustain Princeton's leading position as a center of studies in ancient philosophy in both North America and the Anglophone world.
The situation for students interested in ancient Greek and Roman philosophy has changed a good bit in the last ten years. The top departments have always had multiple scholars working in the area. At this point, this would mean the top choices are, besides Princeton, University of Chicago (Agnes Callard, Michael Forster, Gabriel Richardson Lear, Jonathan Lear, Martha Nussbaum), University of Toronto (Rachel Barney, Lloyd Gerson, Brad Inwood, Mohan Matthen, Jennifer Whiting), and perhaps Yale University (Susanne Bobzien, Verity Harte, Barbara Sattler) and University of Texas at Austin (R.J. Hankinson, Stephen White, A.P.D. Mourelatos [though he has been on unpaid leave a fair bit and may be phasing into retirement--he is in his early 70s], and Paul Woodruff [though he is now Dean of the Undergraduate College]). Chicago students also benefit from an inter-university consortium with the University of Chicago, University of Illinois at Chicago (Constance Meinwald), and Northwestern University (David Ebrey, Richard Kraut).
Other major departments with a strong commitment in the field would include Cornell University (Tad Brennan, Charles Brittain in Classics, and Gail Fine, though she has a good deal of leave currently, which she spends in Oxford, where her partner, Terence Irwin, is now the Professor of Ancient Philosophy), Rutgers University at New Brunswick (Robert Bolton, Alan Code), University of Arizona (Julia Annas, Rachana Kamtekar), Columbia University (Wolfgang Mann, Katja Vogt), University of Pittsburgh (James Allen, James Lennox), University of California at Los Angeles (Sean Kesley, Gavin Lawrence), and Stanford University (Christopher Bobonich, Reviel Netz in Classics). One school to watch in this regard is New York University, which has one ancient scholar on tenure-track (Matt Evans) and Philip Mitsis in Classics, as well as a senior offer out for a scholar of ancient philosophy (and additional faculty joining in Classics).
UPDATE: The point of the preceding was to talk ONLY about the situation for study of ancient philosophy in North America--sorry that wasn't as clear in what I wrote as it was in my own mind! Oxford probably still dominates the field for those looking outside North America, and many other U.K. programs are very attractive: Cambridge of course, as well as King's College, London and St. Andrews/Stirling, among others. In any case, we shall have a new set of surveys on this issue shortly.