If one were to make precision flat parts, what is the material of choice? I’m looking into granite at the moment. Is there anything lighter? Perhaps something composite actually. This is not for an autoclave process.
Toughened glass. We use it for all flat panels which are infused.
Nothing more flat than glass.
that depends on the supporting structure though! Glass is flat, but can bend if not supported well in the middle.
I think most materialism can achieve flatness, but it will have to do with machining it TO the flatness required. Aluminum, steel, carbon, etc…as long as your machining is within tolerance, then the part will achieve the same. If using a large area, and a relatively thin mold surface, you might need to invest in some sort of micro adjusting mold base/support. Then you can measure the flatness and adjust any parts of the mold that sag.
Toughened is actually the worst glass for accuracy, it is annealed (heat treated) and is often slightly rippled. Ordinary float is near perfect.
By flat I mean accurately flat, deviates from a plane less than something like .002 in over so many feet. Not necessarily smooth.
I’m sure you can get a machined steel plate done by precision places. but of course, it must by accurately supported so it doesn’t sag.
I’ve looked into it abit and it seems granite is the material of choice.
Granite is hard to beat. Super cheap and very flat.
You must not have a precision granite table. 
We had a medium sized one a while back that floated on an air cushion, and aside from being several $k, it could only be moved with a fork lift.
Glass is a much better surface to mold from.
There is a lot of cheap Chinese granite flooding the market now. Also e-bay has granite surface plates for pennies on the dollar. I don’t know what the OP is buidling, but if it is small then a small plate is pretty inexpensive.
You do have a point about weight though.
I make bicycles. They don’t require 0.0001" flatness, so no I don’t have an ultra precision surface plate
Oh I forgot, all those in Southern California can check out some cool bikes next weekend at the San Diego Custom Bicycle Show. There will be 3 or 4 carbon fiber manufacturers there. It is pretty interesting.