Worst one for me at the moment is Windows 10. Most of Windows 7's notification noises are pretty short, quiet and sweet, but for Windows 10 they decided to create about 20 incredibly similar yet bland "bingy-bong" sounds and attach them to every single possible event imaginable.
Result is a workplace chock-full of "bingy-bong" "bong-bingy-bong" "bingy-bingy-bing-bing" etc. sounds firing out all over the place. Gah!