We loved to stay at Merc place, the apartment is very well located, in the middle of Ribeira, with easy access to sightseeing, restaurants, cafes, etc. within walking distance.
The place is well decorated and it has all the amenities you might need:)
Eva, Luana and Dani were very helpful and provided good tips for us- I would definitely recommend the place to my friends and I would come back, should I return to Porto at any point! :)