The best are Bicicleterías CANAGLIA, the stores have a web site. But whatever be the bike store you go, be carefull with the price of the products, remember that one american dollar is three pesos (U$ 1 = $ 3.10 or 3.20). Some times is advantageus purchase all the staff in your country an not here.
Canaglia is the most popular store, but definitely not the most "pro." Depending on what you need (service, parts, etc), i would try Lauro Bicicletas (lots of cannondales and newer stuff), Rodados Diaz (for that old world, campagnolo feel), or Circuito KDT to do some laps and get your bike serviced.
i pretty much went to every bike shop in BsAs and found those 3 to be the only ones worth going to.