Reconquista turns into a mob scene. In fact, instead of blocking off the streets like they've done in the past, this past year they've turned the street into a pedestrian street.
There's often drinking in the street, but you have to pay as much as AR$40/person or more for a door charge (but I believe you get that much credit for inside) just to enter the bars. The biggies like Killarney are overflowing, but there are some others that you can get into if you care to.
There is also the TEMPLE BAR which stands alone on Marcelo T. Alvaer between Suipacha & Pelligrini that absolutely overcharges like crazy.
Other expat bars like EL ALAMO (Uruguay b/n Santa Fe & Arenales) will have their own events as well as hope for overflow from SHAMROCK (Rodriguez Pena b/n Arenales & Juncal) which is obviously an Irish bar.
In Palermo, look up SULLIVANS (on Borges 1702 @ El Salvador) if you want an Irish bar or SUGAR (Costa Rica almost to Armenia b/n Armenia & Gurruchaga) if you want cheap drinks (no Guinness) in the Palermo area.
Also, Plaza Serrano, and some of the bars in Palermo Hollywood will probably be somewhat crowded as well.
I don't have suggestions for San Telmo, but there are probably some things going on.
But the biggest blowout is definitely on CALLE RECONQUISTA. I did that last year and I don't think that I'll do it again.