My guess would be the Faena too, but it's much more expensive than most Ws.

There are a bunch of interesting boutique hotels in Buenos Aires, but most of them wouldn't have a bar scene.

Perhaps the OP should think in terms of picking a hotel with good bars catering to young people nearly. To me, that would suggest the Palermo/Soho areas, but I'm sure there are experts around here who can make suggestions.