As mentioned in my question I am looking for a sunset spot in Porto. That could be something like a beach or park with good view of the sunset to have a glass of port wine from the supermarket and take some pictures.
I guess it depends if you want a photo of the sun, the city or the bridge/river.
I remember going across the bridge from the old city, there's a bit of a look-out. It's pretty amazing. So let's do some searching:
Googling for sunset porto gives some pretty good shots. If you want a river shot, consider getting on a boat and timing it to be below the bridge at sunset.
The cliff back behind the bridge looks pretty good too.
Finally, even just standing on the bridge, looking down the river either at the river, or back towards the city would produce some amazing photos.
I don't want to copy people's pics, but I'll try and dig up my old ones and see if I had some too when I get home.