What a wonderful question!
Personally, I can't sing the praises of the Donegal beaches enough and it's so very hard to pick just one part of their coast. So, if you don't mind terribly, I think I might just list two of my absolute favourites. Both are in Donegal, but just on very different sides of the Donegal coast.
The first is Malin Beg, and it's nearly impossible for me to put it into words. The curved beach of the makes it feel like its own little world, with just a brilliant blue sea and calming waves. One part of this beach that steals my breath every time is how the waves roll in, each a perfect ring.
Next is very different. it's Rosapenna, which is about two hours north of Malin Beg. Unlike the sheltered beach above, this coast is open and looks nearly endless. It's such a pleasure to just walk along the sand and see the mountains in the distance. It's an absolute experience, even.