I second Gaucho.
I have used this company 3 times in the last couple of years and the apartments are exactly as you see them on line AND the service is just perfect. They are always on time, very honest and responsive and very helpful.
http://www.buenosaireshabitat.com/
One caveat: there are tons of cheaper apartments. You truly get what you pay for. The cheaper apartments usually have no view, are much smaller with not so plush furnishings. But the pictures are true and the descriptions are exact.