I would go for the bigger chains like NH hotels, Melia, etc. I think Sheraton is probally within reach as well and with the international brands you can expect what to get which is not always the case with local hotels
gregbrophy, factor in VAT to be added to listed room charge and if you have a moment look over the bookassist.com and fodor.com ("buenos aires" as keyword/s). There is always the Four Seasons and Sheraton. Your folks will love their spa. Guaranteed!