The best gym in Buenos Aires is the Megatlon in Barrio Norte. For anyone who knows anything about Buenos Aires gyms (gimnasios), the name Megatlon should be familiar. Megatlon is the largest chain of gyms in the city. There are more then 20 locations and, in general, the facilities (and prices) far outpace all of their rivals.
The thing to know about Megatlon, though, is that not all their gyms are created equal. The one in Villa Crespo is in need of a major upgrade and the relatively puny one in the Alto Palermo shopping mall really has no excuse for charging the prices they do.