What is the best place to hook up with a regular Argentine girl to find as a girlfrie
It is not hard in BA to hook up with good looking women. Especially if you like them in their 30s-early 40s. If you see one, and you smile at her, she will smile back if she is somewhat interested. In the Patio Bullrich and Florida areas you might find some English speaking ladies. Speaking some Spanish will help but showing interest to learn will help also.
Ask questions, flatter her about her looks (without lying) and it is very likely than by the second date you will be kissing and more.
Places to meet interesting women: the cafe / bar across the street from the University of Law (Universidad de Derecho) on Avenida Libertador near Callao;
La Paz (more progressive types); the museums and art galleries are good places. And don't act like an ass. Be a gentleman and you will score.
Hope this helps.