We loved Diogo & Cousins's apt. It was very spacious and comfortable, with a fully equipped kitchen and a spacious bathroom counter to put all your toiletries. The apt is located in the gorgeous and bohemian Alfama neighborhood yet very close to Baixa/Chiado area and station. There are plenty of restaurants, cafes and pastry shops nearby.
The pictures on the app don't do it justice. The lighting in the bedrooms is beautiful, provided by big windows that can be fully closed for a good night sleep.
You will be in a very centric area, 2 mins away from a bus that takes you to the best "mirador" and one of our favorite places Castello San Jorge.
The only downside, if so, would be the set of 3 steep stairs on your way to the apt, but you'll get used to go up through hills pretty quickly, since Lisbon is full of them.
I would definitely recommend this apt for up to 4 people.