You're actually spoilt for choice in Devon - it's a huge county with two coastlines, so lots of options.
I grew up in East Devon, our beaches of choice were Sidmouth, Beer, Branscombe, Budleigh Salterton, Lyme Regis (technically Dorset I think), Exmouth (though that's a bit rougher lol), but most of those are pebbly, and although lovely, probably wouldn't be the first place I headed to in Devon if I didn't know where to go.
I adore the S Hams (Salcombe/Dartmouth area), lovely small beaches and coves, a bit posh, quite boat-y crowd.
N Devon has some stunning large beaches, as recommended above, Braunton, Saunton, Putsborough, Croyde, Woolacombe.
Now, if you want adventurous walks as well as beaches, consider S Hams for Dartmoor, and N Devon for Exmoor.
I don't mind Torbay, but wouldn't choose it above the other areas recommended.
Devon's lovely, make sure you get to Exeter as well (although I'd skip Plymouth).