I'd use 3/4" AB granite. This is a mixture of size from dust up to about 3/4" pieces. The material pieces pack together tightly to form an almost solid surface. If you think that water can get in, you might consider crowning the surface so the water will drain to the outer perimeter. I'd scrape away at least the grass surface beforehand. After spreading the material, consider compacting it with a roller. During the first couple of years, if any weeds come up, apply Roundup. As you continue to drive tractors across it, it will pack to the point that weeds have a hard time getting roots down.
Now if you can afford it, I'd recommend paving the surface with asphalt (concrete's too expensive and chip-n-seal too rough surfaced). With a smooth surface you can custom build a dolly for each implement. It is amazing how easy and fast tractor implements are to attach to the 3ph when you can roll them right up to the tractor and make minute adjustments with just the nudge of a pinkie. The dollies are just lumber and casters, so really inexpensive. After all you've invested in the barn, the cost of asphalt might not be that much more to turn this into a very time and labor saving project.
One more thing to think about is suspending a hoist (elec. if you have power, otherwise, chain) from the center of the structure ceiling (assuming strong beams). Then you can lift implements if needed, or an engine if you need to pull out & overhaul, etc.
