The guest house was exactly what I needed for my stay in Adelaide. Pick ups for tours were right across the street. The place was clean and mostly quiet. I was in room 10, which is right by the bathroom and kitchen, so if people are up super late, then you can hear them from that room. Otherwise very clean place, helpful staff, true free wifi and a friendly atmosphere. (Oh and very safe too!)
Students & Travellers Inn is ideally situated in Plaka and conveniently close to Syntagma Square. They have the typical conveniences of most hostels (computers, free wifi, luggage storage, etc.), but also have excellent staff you greet you with a shot and make sure they answer any and every question you may have. The rooms are clean and spacious, and for those who care, the showers have amazing water pressure (compared to many others throughout Greece).
The Urkmez Hotel was perfectly located and the staff was excellent. I would say that the rooms are a little on the smaller side, but this is hardly unusual for Europe and not something to complain about.