I recommend Hospital Aleman. I have been a member for almost two years now and am very happy with my service. They have a central hospital, but have deals with clinics all over the city where you get the same level of care with the same deals. I've been out to clinics out here around Pilar that are fine.
I pay about $550 pesos a month for full coverage (I don't remember what my plan name is) for my wife and I.
I have not looked into getting extra coverage for international situations yet, so I can't speak to the cost of that, but I do believe they have it available.
Swiss Medical is another good company. It's a bit more expensive than HA (not too much more), but I know people who have it and are happy with it as well.
When choosing a health care company in Argentina the main aspect to consider is in which clinics or hospitals it covers hospitalization or ER: that is where the costs are really expensive if you're not well covered. It should cover hospitalization at least in the Swiss Medical clinic, Mater Dei clinic, Otamendi, Instituto del Diagnostico, Hospital Austral, Fleni, and I'm sure I'm leaving some good ones out.
If the plan you choose has good hospitalization, the specialists that are included are also, most likely, good ones (English speaking is a completely different story).
Try to stick to the major health care companies: OSDE, Omint, Swiss Medical. Hospital Aleman has been mentioned (and recommended) many times in this forum, but I have never been there personally or know their plans.