Out of a total of 531,704.12 acres of vineyards (2019, Argentina), this is the ranking:

Malbec
109,685.95 Acres
38.60% of red varieties
22.39% Total cultivated area in the country

Bonarda
45,233.24 Acres
15.92% of red varieties
9.23% Total cultivated area in the country

Cabernet Sauvignon
35,284.34 Acres
12.42% of red varieties
7.20% Total cultivated area in the country

Syrah
29,444.64 Acres
10.36% of red varieties
6.01% Total cultivated area in the country

Tempranillo
13,742.78 Acres
4.84% of red varieties
2.81% Total cultivated area in the country

Merlot
12,740.99 Acres
4.48% of red varieties
2.60% Total cultivated area in the country

Pinot Noir
4,970.37 Acres
1.75% of red varieties
1.01% Total cultivated area in the country

Cabernet Franc
3,109.88 Acres
1.09% of red varieties
0.63% Total cultivated area in the country

Source & Credits: Wines of Argentina

So the Malbec is the undisputed champion here.

Here is the little story about how the Malbec grape was saved by Argentina (Governor and later President Domingo Faustino Sarmiento)

Risposta di Leopoldo Fernandez Suarez a Se trasferiamo un albero dall'Europa che cresce le foglie in primavera in Argentina, crescerà le foglie in autunno o in primavera?

Credits: winesofargentina (.org)