This is a very tough call. I love Chicago, as it is such a huge part of the early Macintosh UI (also the early iPods). New York was my default word processing font until OS X became my primary operating system. All of those old Susan Kare fonts were special in their own way. However, I'm going to go with Bill Atkinson's Venice here. I always used this as a heading font and have yet to find a font as beautiful for that purpose.