Try before you buy. You ask 10 operators what the best rig is and you'll get 20 different answers. What features do YOU care about?
Tiffen is very good at all in one rigs with lots of power and lines down the post. The built in super posts, tilt heads, and motorized stages are handy on some jobs as well.
PRO is drop dead simple as others have said. You can clean their arms with a garden hose if you need to and you can build a sled in parts.
XCS is rigid and can be just as reliable as Pro, but more features can mean more problems.
There's no best overall, there's best for you. Steadicams all work on the same principle and physics. Everything else is just gravy.