I have never coached this event, so I do not speak with great authority.
My personal thought for it this year was a perfectly round marble or ball bearing. For this I would have used 1/2" PVC pipe split the long way. With just the right amount of heat, it can be gently bent. A large hair dryer might work. I have an industrial size one, so I might be biased. Actually, boiling water might be able to be used with the proper jigs and safety equipment. If the ball was rolling fast enough It could be bent on a generous radius to at least 90 degrees. This would be slightly banked like some road curves are. You could use sweeps for the curve but it will use up a lot of space. It would need to be split the long way.
PVC pipe is nice to use as you can take a 10' section and run the marble thru it with different slopes to get a good idea of how much ril it will take to get certain times. Adjustable slopes on your final product can go a long way to having very good times for the run.
Hard wood with a very generous "V" slot cut into it with a table saw or preferably a router is an option.
If you used wire, it would require good "railroad ties". It would need to be quite stiff so that it did not accidentally get bent. You might be able to use 1/4 copper tube. Metal rails would really need to use a proper bender on it. I have a very old one that is now rather expensive that makes nearly perfect bends. There are cheap to moderately expensive hand bending tools. Look at copper tube bending tools. The ones that look like a coiled spring that fit over the outside of the tube are rather cheap, but there might be a nasty learning curve.
Strips of 1/4" PVC could be screwed/glued to a wood base of a proper width for you ball. You would probably have to glue PVC side boards that go about ball height above the "rail" that it runs on to avoid "OOPS"
Another option would be the blue "smurf" plastic electrical conduit This can be readily hand bent. You might want to glue it to some 1/4" sheet PVC pieces to maintain curves or as mounting brackets.
There is no best option! It is what ever you can get comfortable with. Start practicing early on what will become scrap material.