Buenos Aires enjoys tourists flocks all year round. If you want to avoid especially large crowds, don't come during holidays, like Christmas, Easter or European summer vacation period. Shoulder seasons—spring and fall—are a good time to visit the great city. Mild temperatures will let you enjoy your time outdoors and observe nature.
September–November period offers the most reasonable hotel rates. And the weather will be warm and sunny all the time. From December Buenos Aires sees more and more tourists up untill February, which is a peak season due to Carnival. After that you can enjoy a calm season in the city again. Fall foliage and good weather will stay until June that will bring rainy gloomy days.