You really can't go past City Beach - for such a popular spot, it's surprisingly peaceful, and a spot on the grassy hill behind the beach is perfect for a picnic. You can see all the way to Rottnest. Absolutely stunning.
Although I have not done this for a long time, I would have thought most of our beaches from Hillarys down to South Fremantle would all be great spots.
I prefer South Fremantle I think it is called, as you can take dogs.
We are lucky, here in the West, that we have such a choice of lovely beaches and therefore, lovely sunsets at so many.