Given the size of Argentina, you’re unlikely to flit from region to region and, if you can, you should try and visit each area at the optimal time of year. Roughly falling from September to November, spring is the best time to visit Argentina, although in the far south icy gales may blow. Summer (Dec–Feb) is the only time to climb the highest Andean peaks, such as Aconcagua, and also the most reliable time of year to head for Tierra del Fuego, though it can snow there any time of year. Buenos Aires is liable to be very hot and sticky in December and January, and you should certainly avoid the lowland parts of the North at this time of year, as temperatures can be scorching and roads flooded by heavy storms. Autumn (March and April) is a great time to visit Mendoza and San Juan provinces for the wine harvests, and Patagonia and Tierra del Fuego to witness the eye-catching red and orange hues of the beech groves. The winter months of June, July and August are obviously the time to head for the Andean ski resorts, but blizzards can cut off towns in Patagonia and many places close from Easter through to October, so it’s not a good time to tour the southerly region. Temperatures in the north of the country should be pleasant at this time of year, though Buenos Aires can come across as somewhat bleak in July and August, despite a plethora of indoors attractions.
A final point to bear in mind: the national holiday seasons are roughly January, Easter and July, when transport and accommodation can get booked up and rates are hiked, sometimes almost doubling.