What's best to use in a sawzall to cut steel pipe?
It will probably be 3 to 6 hours of non-skilled labor (neighborhood kids?) to dig up the concrete and remove the whole assembly.  Don't cut the post off:  it will help wiggle out the base.

Dig a hole immediately to one side of the concrete base of a convenient size.  (There is no need to dig all around it.)  A post hole digger works great.  Make the hole at least as wide (front to back) as the concrete so you can tip the concrete into the hole.  The hole need not be as wide (left to right) as the concrete; only enough to detach it from the surrounding soil.  The concrete depth should be at least 36 inches below the frost depth, but that is hard work and most people would go 24&ndash;30 inches and call it good.  (Also note that link says a standard parallel-sided hole is not desirable, but most installers use an auger or post hole digger so it is unlikely to be pyramid shaped.)

The block of concrete will weigh up to 250 pounds, so no need to be polite and dainty when trying to move it.  Roping it to a heavy vehicle, tree, boulder, etc. and jumping on the line would not be overkill to gain sufficient force.

Once it is out, use a sledgehammer or blunt end of a splitting maul on the concrete to break it off the pipe and into easy-to-carry and easy-to-use pieces for trimming a garden, driveway fill, etc.  Then you'll also have a sturdy pipe to build a fountain, chime, etc. too.