Beauty is surely in the eye of the beholder and thus there can be no specific No 1. Answers already recorded prove this as all are truly beautiful in their own but differing ways. Scandinavia, UK, France, Iberian Peninsular, Australia, New Zealand etc, etc amongst others all have coastlines in places that take the breath way. The most beautiful will differ according to the observer and maybe the time or circumstances obtaing at the time of viewing to the individual. As an individual myself, I could not nominate one single coastline for fear of betraying another. At the same time, those who nominate specific coastlines provide impetus to visit those indicated that, as of yet, ve not been viited by myself.
I have visited many other countries and observed many beautiful coastlines..however..when I feel the urge, it's not like I can hop in the car and drive there for a day/week-end..and that is why the west coast of Florida is my favourite..when I get the urge I jump in my car and bingo I'm there, sitting on the warm white pristine beach ( 80 steps from our beach cottage ) listening to the surf nd the birds..enjoying a cold drink..contemplating or meditating..prioritizing .. my life...so I have to say " beauty is in the mind of the beholder "..thanks for asking a great question...seize the day !
Coming from the UK, I think many people don't realise England has a beautiful coastline. Cornwall has amazing beaches, yellow sand and turquoise sea in some places. Looks tropical, aside from the cold temperatures. St Ives is beautiful.