Used Buenos Aires Bus on my recent trip.

It was pretty good for a general tour of the central areas, map of the route is provided and each seat has a set of headphones with commentary provided in 10 or so languages.
We didn't get off anywhere as we were quite late starting off.
The starting point at Florida/Diagonal Norte gets very busy so I'd prebook a departure time if you are able.
Doing a one day tour was plenty, given how easy it is to get around the city can't see why anyone would want to spend two days on a tour bus.