So hard to pick just one! If we're going favorites (though maybe not best, exactly), I'll have to go with the miniseries The Prisoner. I also have to mention Frasier, as it's my favorite classic sitcom.
I just finished watching through The Mary Tyler Moore Show and recommend it to anyone who enjoys sitcoms. Basically every sitcom since its airing bears some influence from it.
Still need to watch I Love Lucy, but obviously that one is the grandmama of all sitcoms. I mention MTM because it's not nearly as remembered, but it should be.