For me, the first two interviews I ever did were the best ones of all time. They pretty much all went downhill from that lofty standard.
First one was actually an interview for a guy who was coming in for an unpaid internship. Dude walks in wearing a suit. He was literally dressed better than I was. Turned out to be absolutely professional during said interview. In fact, he was better in this interview than some of the people I interviewed for a full-time reporting gig.
Second one was the first person I ever interviewed for a full-time job. The freelance photog I mentioned in the interview duds thread was using the lunch room where I was planning to conduct the interview. Ad director was in his office, etc. I finally ended up using the circulation manager's office to conduct the interview. All the while, I'm thinking this sends a bad message to the guy I'm about to interview but he's taking everything in stride.
I was really nervous because it was the first time I was interviewing someone for a full-time position. However, the guy put ME at ease before too long. He wasn't wearing a suit, but he had the shirt, die and dress slacks thing going, but the thing that struck me was his answer to one of my early questions. He said, "I'll stay up 'till one or two o'clock to make sure a story's right." More significant, though: He was the first person I asked a question that's now my signature interview question ("why did you pick the clips you picked?"). Every other time I've ever asked that, I asked it with the mindset that it's an interview question and I'm going to use the answer to help me decide your fate. When I asked this guy that question, I felt like I could have asked him as we kicked back with a couple of beers at the bar.
Needless to say, this was the guy I ultimately hired.