The buses are a very good public transport, one of the best of the world. You can go in bus practically everywhere, there is always a line that leaves you a few blocks away from your destiny for a few bucks. Sometimes they are crowded or slow (but then all the traffic is slow)

Prefer the subte (subway) when possible, but the grid is limited. It is cheap also.

The taxi is cheaper than in other places on earth, but still much more expensive than bus or subte. You will be safer calling a radio-taxi company when possible.