Pascal, the boring tool that just keeps working for decades. I wrote a program to control a large water meter calibration system in the 1980s, using TP7 on MS-DOS. I did graphics and multitasking, along with monitoring the pumps, flow meters, etc.

I was contacted in 2004 when it stopped working... because they had inadvertently swapped the two IO cables when replacing the PC it ran on. Y2K had zero effect on it.