Argentina / Uruguay·~45 millones
Río de la Plata (Argentina + Uruguay)
El más distintivo de todos los castellanos. Italiano fonético, vos en vez de tú, y el 'sh' por todas partes.
Pronunciation
The famous 'sheísmo' or 'zheísmo': 'll' and 'y' are pronounced /ʃ/ or /ʒ/ — 'me llamo' sounds like 'me shamo'. Italian-influenced melodic intonation, especially in Buenos Aires (the porteño accent). Seseo throughout. Strong rhythmic stresses.
Grammar and register
VOSEO: 'vos' replaces 'tú' completely. The verb conjugation shifts too — 'vos tenés' instead of 'tú tienes', 'vos sabés' instead of 'tú sabes'. The imperative also changes: 'vení' (vos) instead of 'ven' (tú). Plural is 'ustedes' (no vosotros). Imperfect subjunctive uses '-ría' forms more than peninsular Spanish.
Distinctive words
hey / dude (the iconic Argentine opener)
“Che, ¿qué hacés?”
dude (affectionate among friends, insult otherwise)
“¡Boludo, qué hacés acá!”
work / job (from Italian lavoro)
“Mañana tengo mucho laburo.”
kid / young person
“Ese pibe tiene futuro.”
mess / chaos
“Esto es un quilombo.”
the traditional herbal drink + the gourd to drink it
“¿Tomás mate?”
Context
The Italian immigration of the late 19th / early 20th century reshaped both the vocabulary (lunfardo slang from Italian) and the intonation. Tango. Maradona. Borges. The fact that Argentine Spanish is the LEAST mutually intelligible with peninsular Spanish for first-time listeners is a quiet source of national pride.