David Gergen -- The Best President to Have a Beer With Is ...

Legendary presidential adviser David Gergen has worked for 4 different administrations, so who better to ask ... who's the best Prez to grab a brewski with?

Gergen worked alongside Richard Nixon, Gerald Ford, Ronald Reagan, and Bill Clinton ... which is why our photog in D.C. put the suds question to CNN's Senior Political Analyst on Sunday.

Gergen couldn't pick just one, but narrowed it down to two ... and he had good reasons for each of his choices.