There are many things that distinguish Lisbon from other European capitals, the spectacular light, being close to a very large river and the sea (Tagus River and Atlantic Ocean) and having many sunny days, are some.
These are probably the most obvious reasons why Lisbon's famous sunset is wonderful.
Because of the movements of the Earth, the sunset is not the same place every month of the year and for that reason the place where the sun disappears from our view changes a little every day.
In my opinion, September and October, some years also November, are my favorite months to watch this phenomenon. They are those in which this moment of the day becomes even more spectacular.
There are also many places where we can observe it, but not all are the same, for example, near Lisbon you have wonderful spots:
Boca do Inferno - Cascais.
Fonte da Telha – Caparica.
* Photo offered by FT Beach Hostel
If you are in Lisbon there are 3 places you shouldn't miss:
1 - Cais Sodré
Next to the Tagus river, on the right side of those facing the river terminal.
2 - Chapalimaud Foundation miradouro - Belém.
3 – Cacilhas.
Go by boat to the other side of the Tagus River (Cacilhas) and sit in one restaurant (Ponto Final and Atira-te ao Rio) or even on the ground near the river.