If you include third-party applications, I'd have to say Excel was the coolest. Excel v1.0 was released for the Mac in 1985 (the year after the first Mac). Excel wasn't ported to the PC until v2 in 1987.
I remember my first encounter with it about that time and thinking that it might be a pretty useful tool. Little did I realize how much time I'd be spending trying to make sense out of mountains of test data using it!
Other than various email applications and web browsers, Excel is probably the application I use the most for my work and it features pretty prominently here in the FIRST community for
engineering and
scouting .
Running a close second, Ashlar Vellum introduced the world to parametric 2D design on the Mac. At that time it was competing with AutoCAD (v6), MacDraft and Claris CAD for the low-end engineering design market. I used it in 2D, 3D and "solid modelling" versions until Inventor came along. Now if Autodesk would only port that baby to OS X...