We think the best time to visit Buenos Aires is late spring (November and December) or fall (March-May). Forget a summertime visit: the vast majority of Argentines take their vacations seriously, and the city is largely empty from Christmas well into February.
Seasons in B.A. are exactly opposite of those in North America: Summer in B.A. is December-February and winter is June-August. In recent years, however, late August and September have been unseasonably warm and pleasant, too.
The climate seems increasingly to be melding into one long warm-to-hot season and one shorter cool-to-cold season. Buenos Aires summers are hot (averaging 82 F/28 C) and can be oppressively humid, though once in a while there might be a cool evening. Winters are mild (averaging 40 F/5 C), but there can be at least a couple of very cold and wet weeks. Rain can occur any time of the year, often as thunderstorms.