There are two types of chassis: monocoque and space frame. The original GT40s had a mopnocoque chassis, and of the manufacturers you listed, only RCR has announced a monocoque, although I believe RF is working on one. Regarding the "frames" you describe, while space frames may look similar at first glance they are significantly different in design and construction.
I will give you my opinion on whose space frame is best. I may upset some people in doing so, but this forum is all about candid exchange of information and opinions. Full disclosure: I am building an RF space frame.
Here is how I would rate the space frames of the manufacturers you have listed: RF > RCR > > MDA > Tornado. I am not certain about the exact order of these, but I am certain that the first is better than the last two. Let's start with the RF. It was designed by John Shepherd, a highly regarded Australian chasis engineer who I believe worked with Carroll Smith and who was heavily involved in racecar design and engineering. If you examine the RF chassis you'll note lots of triangulation of chassis rails to pick up suspension loads and use of larger rails only where needed. RF has published some very impressive torsional rigidity numbers that indicate their chassis is very strong. The RFs typically are lighter than other replicas. I would wager that if you were to model the various available space-frame chassis using finite-element software you would find the RF to be superior.
I have not inspected Fran Hall's RCR chassis yet, but based on Fran's reputation alone, as well as a few pictures, I imagine that it is very well done. I would love to see Fran post some torsional rigidity numbers for his chassis.
I have not yet seen an MDA chassis but I have looked at pictures in some detail. The MDA is a derivative of the GTD, which, after modification, has been race proven in the UK. Like the GTD chassis, the MDA chassis looks to me to use heavier frame rails rather than the RF. There was a recent thread with some discussion about the MDA's surprising lack of a chassis transverse member in the engine area, and speculation that the rear-suspension loads in this area of the chassis could cause this portion of the chassis to flex. The discussion was rather muted out of respect for MDA, but in my opinion it is a reflection of an inferior, even unsafe, chassis design.
I put Tornado at the bottom of the list for a couple reasons. First, the guys in the UK who race GT40s race GTDs, and not Tornados. I think that alone speaks volumes. In fairness, this may be a fluke or possibly due to other considerations of the Tornado car beyond the chassis. I do know that Robert Logan, owner of RF, built a Tornado as his first car. His frustration with the Tornado design was his inspiration for developing the RF. He went out and hired John Shepherd and remedied the Tornado's weaknesses.
There you have it. Mark Sibley and Andy Sheldon and some MDA and Tornado owners and builders probably won't be too happy with my opinion, but so be it. I would suggest that the best way for a manufacturer to prove the worth of his chassis would be to report the weight of the space frame, before sheeting, and the torsional rigidity.
[EDIT - I am now told that RCR will no longer offer space frames, only the monocoque chassis. His monocoque looks very nice, and may be lighter and stiffer than any space frame available. Again, I urge Fran to test and publish the torsional rigidity and weight numbers for his chassis.]