Really close to the city center (around 10-15 min, depending on traffic jam), where you can go by bus (1 bulgarian lev for a ticket), and at 10 min from the airport, the Novotel Sofia hotel is a really nice place to stay.
A modern hotel, with irreproachable rooms, a WiFi access everywhere, and a quite enjoyable breakfast. The food at the restaurant was also nice, but if you want to experiment bulgarian food, there are really nice places in town to visit.
The hotel staff was welcoming and really nice; especially at the bar, where we had really good advice on the drinks, with a special mention for the bartender who masters the mojitos.
A little point : the bulgarian wines were not that present on the wine lists, which might be a pity when we consider the culture of the country regarding this field.
To conclude: it is a really nice place to recommend.