Real: Robert H. Herz, Chairman, Financial Accounting Standards Board. I'd pick his brain all day about the accounting profession, auditing, standards compliance, and other CPA-geek stuff. Then I'd write the most awesome term paper in the history of Accounting 401 term papers. And maybe tell John Cleese he can go fuck himself for the "Vocational Guidance Counselor" sketch.
Fictional: Cmdr. William T. Riker. I'd be his wingman and try to get myself some alien space babe sloppy seconds.