Lisbon is a city of roofs and observation decks. Here is the main secret how to not miss them - to remember the word “Miradouro”, which means “viewing”. The city map of Lisbon is repleted by these Miradouros and you can discover something else just by walking around the city.
- Observation deck on the top of the Santa Justa lift/elevator - 10/10. You can get here by legendary elevator, but there is always the huge que. So another option – to get up via stairs in the mall in opposite side of the street. Then there are 2 options - either to look at the city from the bridge between the buildings, or to pay € 2.5 and go up even higher level of observation deck. Definitely I advise you to pay, because of completely outstanding view on the top. You will enjoy the rooftops of Lisbon for all 360 degrees.
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**Observation deck on LX Factory – 8/10. (**I added 2 points due the bar on the deck =)). View of the 25 de Abril and the statue of Christ. The entrance is not obvious – you should go through the terrace of the Rio Maravilha to the elevator
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Miradouro da Graca - 10/10. Wonderful view of the city and the bridge. The view from here is slightly higher than usual and therefore the viewing angle is huge. Also here in the neighborhood is Miradouro da Nossa Senhora do Monte. Nice place to come after sunset with bottle of wine. Night view is romantic
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Viewing poin (if you can call it like that) near to Capela de Santo Amaro - 8/10 Just because the place looks abandoned. You will find a wonderful view of the bridge «25 de April» here.
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Miradouro das Portas do Sol - 10/10 Portuguese Santorini, no less.
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Miradouro de Santa Luzia - 10/10 Neighbor from the previous viewing and no less beautiful.
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This is not all that I managed to see, but the best ones (personally). I would be grateful if you can recommend other decent Lisbon observation decks in comments 
PS Photos are mine, 3,4 - via Pixel 1; 1,2,5 - Sony Alpha 6000