I have just finished a weeklong stay at the Hotel Lyon (251 Avenida Riobamba, 1025 Buenos Aires, Argentina, Tel 4372-0100. Web site www.hotel-lyon.com.ar Email info@hotel-ñyon.com.ar) and could not be more pleased. My $50 per night room was actually a suite with a sitting room as well as a clean twin-bedded room, plenty of hot water, cable TV, breakfast and some of the friendliest, most helpful front desk staff I encounted on this six-week long trip. (The front desk is manned 24 hours per day.) While here, I lost my passport and they were most helpful, making phone calls for me (my Spanish is poor at best) and offering sympathy.
There is no restaurant, but water and cold drinks are available by room service. There are two computer work stations (free) just off the lobby and Wi-Fi is available in the lobby and the breakfast room. I made my reservation by email.
The Plaza de los Dos Congresos subway station is just a short walk away, fashionable Ricoleta a slightly longer walk. Metered taxis are always available in the street in front of the hotel.
There were relatively few tourists here, so I think it may be an undiscovered jewel.