IMO, Jon Stewart is the best interviewer (especially when he's interviewing non-celebrity political figures).
On most shows, the celeb being interviewed will come out, plug their latest project for a few minutes, and leave. Stewart actually gets them talking about interesting things (politics/current events).
I think what ruined Conan for me was that same opening joke every night since his first show ("we have a great show tonight, guys... I say this every night, and usually I'm lying, but tonight I really mean it."). the only Conan bit that I find really funny any more is Triumph, and even that's getting old.