Porto has many amazing viewpoints from where one can watch great sunsets, but this one is hard to beat: the sun sets in the ocean in splendid fashion while the sailboats passes by and the fishermen interact in lively ways, to the delight of locals and turists alike. Also, the presence of the phare just makes it even more scenic. Picture-perfect!
The sky usually gets even nicer after the sun actually sets, so it's worth waiting! But it can also get cold (normally it's windy), so dress properly!