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.