I likewise have only ever done it once, as most of my characters are hopelessly nice. I sacrificed Golldir, the guy attached to the Hillgrund's Tomb quest. Even when playing a relatively unpleasant character I found it hard to get rid of a follower, but I thought that the way that Golldir let his aunt go into a dangerous ruin all by herself was pretty horrible. He betrayed his aunt's trust, poetic justice and all that.
If I could sacrifice anybody at all it would be Maven Black-Briar by a huge, huge margin. I particularly resent going through that long, elaborate Thieves' Guild questline only to end up feeling like I'm nothing but Maven's chief flunky. I just hate the woman. And I think that after all I've done for the Thieves' Guild, they really shouldn't need her anymore.