I can highly recommend Fredericos place. The location is perfect to explore the city. You can basically walk everywhere. And getting a taxi is very easy as well.
The apartment itself is excellent. It's clean, spacious and well equipped. And like I said the location is just wonderful.
Frederico is excellent host. He took care of us right from the beginning until the end. We arrived early to Lisbon and he made it easy for us to bring our luggage to the apartment so we could go out without having to carry all our bags.