My personal preference is for the AH Spitfire Mk.I. I don't love all of AH's releases, but this time I think they hit it out of the park. The 3D model, the skins, the flight model, and the systems are all solid. Relative to the AH, I think the Flight Replicas has dumbed down systems, with an easier start-up procedure and braking, and a more forgiving flight model and engine management. It's nice, and more accessible, but I don't get excited about the idea of flying it. The best thing about it is that it is a Mk.V, which has not been treated well in flight sims in the past, at least in the Microsoft lineage. I would rank the FlyingIron second behind the AH as it is very solid like all of their warbirds and exhibits enough of the Spitfire's quirks to be interesting.
All of them are good products though, and if you have a favorite variant among the ones covered, that may be a reason to choose that product.
August