I can't think of a single most irritating piece of software that I've ever had to use. But I must say I hate the calculator app on my iPhone. I do things quickly. It's just the way my nervous system works. I have a dozen things on my mind at a time and I want to get through whatever task I'm doing quickly and efficiently particularly if it's a routine thing like doing arithmetic on a calculator.
But the iPhone calculator app won't let me do that. The virtual keys are slow to react and I end up missing numbers and having to retrace by, for example, dividing by the wrong number I just multiplied another number with. And like virtual keyboards on Apple phones, the keyboard on that app is a bit too small for the sausages I have for fingers.
There's a silver lining to this. The calculator app is teaching me the zen of controlling my impulses and doing things more slowly and deliberately. If I can master using this little app, I might be ready for larger challenges!