Personally, I would say that in South America, the best place is were the snow is falling.
High Andes (Portillo, Valle Nevado, Las Lenas) are above tree line, good terrain and good snow quality (not always the most reliable though)
Valle Nevado is big, once it's connected with La Parva and El Colorado with the same lift-ticket.
Portillo is beautiful, and has some good of-pist (not as big)
Further south you will have Termas de Chillan (now called "Nevados de Chillan) which a like a lot, Cerro Catedral, Chapelco, Pucon... and many other smaller resorts.
The further south you go, the wetter it gets and the mountains are not as high... so rain is always a possibility.
In Ushuaia, there is Cerro Castor, the southernmost ski resort in the wolrd, and the longest season in South America.
In my opinion, Chile has better options (however, Las Lenas and Cerro Catedral are nice Argentinians resorts)
unfortunately, lifts are often closed in storms and wind days...