Concrete vs suspended floor over vapor barrier
Is your current setup like one pictured on this site?

What do you mean by "best"? Building a subfloor will be more expensive, but you can probably use your current radon system. Pouring concrete will be a cheaper floor option, but it might not be compatible with your radon system, and refitting it would drive the cost up. 

The wood floor would be the easiest to do yourself if you know enough about carpentry to build a deck.