For best location choose the Plaza Marriott. Right on Florida at Plaza San Martin. It's pretty old but well kept. The Hilton is the newest and arguably the nicest. A little out of the way but with very cheap taxis that's not a major challenge. There are two Sheratons: El Libertador nicely located within walking distance of shopping area but not spectacular. The older Sheraton Buenos Aires is down the hill from the Plaza Marriott at the other end of Plaza San Martin across from Retiro train station. All have pools.
If location is paramount I'd pick the Marriott, otherwise Hilton.