Just like other nations that have a vast terrain, Argentina is as diverse as it gets. Thus their temperature varies in different parts of the country. But many suggest their springtime (October to the middle of December) as the best time to travel to Argentina. You will avoid the tourist crowds and the high prices of the peak tourist season.
The central part of Argentina where Buenos Aires is located is quite comfortable at about 18°C with their warmest months in December to February. The southern part of the country has places like Argentine Patagonia and Tierra del Fuego. The best time to visit here is from December to March. But either way, the best time depends on where you want to go and what you want to do.