Best time of the year to visit Buenos Aires

Buenos Aires is in the southern hemisphere, which means its seasons and temperatures are very different from North America, Europe, and most of Asia. Here's what each season is in Buenos Aires:
Spring: September-November
Summer: December-February
Fall: March-May
Winter: June-August
All things considered, September-October, March-May are the best time to visit Buenos Aires. The key considerations are temperature, rainfall, and timing of the tourist season. More details below
In terms of weather, the best time to visit are between late Spring to early Fall, basically October-April. The day time temperatures during these months are all above 20C (68F), so is very comfortable to walk around the city in something light. It can get pretty hot in the summer, going up as high as 30C (86F) in January, so if you're not good with heat you should avoid December, January and February. Here's a what the temperature in Buenos Aires looks like across the year:
In terms of raining, the summer months are generally wetter with more intense rainfalls and thunderstorms, but the frequency of rainy days is about the same throughout the year, with about 1/3 of the days being rainy days. Also, in the summer you get longer days so you get more sunshine overall. Overall, I wouldn't say rain is a huge issue for travelers. You can see in the chart below, October - March are the wettest months of the year
Finally, in terms of tourist season, the summer months, especially November - February, are typically the months with the most number of tourists and therefore the highest prices for hotels and flights. Ideally you will want to avoid visiting during this time to avoid the crowds and high prices