I would highly recommend S1 by Athens Suites. It is exactly as advertised (better in reality actually!). It is an easy 3-4 minute walk from Monastiraki (direct train from Airport) or Thissios subway stations. Check-in is so simple (key card entry at front door so even if you arrive at 2AM there is no issue with getting into the building). The suite itself is extremely spacious, sparking clean, well appointed with a kitchen and fridge, etc. Kostas and Eva are extremely helpful (they even printed out my boarding pass for me) and have tons of suggestions for what to do. S1 is smack bang in the middle of a really cool area (Piris which is known for cool bars, restaurants and city famous graffiti), a short walk (took me about 12 minutes) to the Acropolis, Ancient Agora and all the other sights. I will definitely be staying here again!