From Brian Leiter (Texas):

1. Michael Graetz (Yale): 470 citations, age 63
2. Daniel Shaviro (NYU): 400 citations, age 50
3. Edward McCaffery (USC): 340 citations, age 49
4. Joseph Bankman (Stanford): 320 citations, age 52
5. Reuven Avi-Yonah (Michigan): 290 citations, age 50
6. David Weisbach (Chicago): 280 citations, age 44
7. Edward Zelinsky (Cardozo): 270 citations, age 57
8. James Strnad (Stanford): 260 citations, age 55
9. Anne Alstott (Yale): 240 citations, age 44
10. Lawrence Lokken (Florida): 240 citations, age 68

Six of these Tax Profs also are ranked in the latest SSRN Tax Faculty Rankings:  Shaviro (#14), McCaffery (#6), Bankman (#24), Avi-Yonah (#7), Weisbach (#9), and Strnad (#23). Leiter continues:

There were several runners-up to the top ten here, whose citation counts were awfully close to the top ten: Robert Peroni (Texas) with 230 citations; Marjorie Kornhauser (Arizona State) with 220 citations; Alvin Warren (Harvard) with 220 citations; and Lawrence Zelenak (Duke) with 220 citations. Some scholars, of course, work across different fields. My colleague at Texas Mark Gergen also had 290 citations, but these were about evenly divided between his tax scholarship and his private law scholarship. So, too, Kyle Logue at Michigan had 250 citations, but many to his work on torts and insurance, others to his tax work. No doubt there are other scholars in a similar situation.