Hotel Vienna was renovated recently, has a great locatIon near metro Omonia. You can also reach center Monastiraki, in about 15 minutes walking. You can also reach Gazi in 15-20 minutes but foot. The staff is very friendly, good breakfast included in the room price, free internet pc and wifi, very good price.

we stayed 4 nights in September. The receptionist was very helpful, arranged us some day tours at a discount. The breakfast is simple but typical, bread, coffee/juice, jam, cream cheese.
the location is 2 blocks from Omonia metro station, and 20 min walking to Syntagma. We mostly walked. The rooms were cleaned every day.

We have stayed in many 3 star hotels in Athens and this was definetly the most cleanest and with excellent reception staff & manager! The dangerous Omonia square is very exaggerated. Its true that there is a lot of immigrants, but we didnt feel scared. If you have traveled in any big citys as London, New York, etc... its no different.
The breakfast is a basic breakfast basket, not a buffee, but you get coffee & croisant =)
The reception manager was telling us stories from greek mythology and helped us to go in the beatiful lake vougliameni!

The rooms here are pretty basic. The staff of the other hand are fantastic. I have never seen more helpful hotel staff ever. Any questions or concerns will be handled immediately. The hotels website also gives you directions on arriving from the airport. As previous posters have stated, the area isn't great, but just keep to yourself as your travel back to the hotel at night. There are lots of food options close to the hotel. Walk out the hotel and turn left towards Omonia station. Close to the McDonalds is a great place for gyros and rotisserie chicken. There's a grocery store close by as well. Ask at the front desk for directions.