Without a doubt, Lotus 123. Now, it wasn’t perfect, and as far as spreadsheets go, they eventually mucked it up (I abandoned it for Borland’s Quattro Pro which still does 3 dimensional spreadsheets better than any other program). But Lotus opened my eyes to computers. I was never a numbers guy, but I learned that I didn’t have to understand numbers to be good with them, I just needed to learn how to talk to the computer, and cajole it into doing my work for me! This led to hardware (how to set up a RAM disk to make things work faster), operating systems, programming, and everything else. I’ve loved using many other programs (not ‘apps’, they’re programs!) since then, but Lotus was the genesis of it all. And I can still write Lotus keystroke macros in my sleep!