Thanks, CDavis, I'll attempt an answer to Lil Dragonfly's question here.
Between 1942 and 1949, BBC Radio broadcast a series called [ame]http://en.wikipedia.org/wiki/The_Brains_Trust[/ame]. One of its panel of experts was
Cyril Edwin Mitchinson Joad, who frequently prefaced his answers to difficult questions with the phrase "it depends what you mean by ..."
To the question "what are the best rubber fins?" I'm going to emulate Joad and respond: "It depends what you mean by 'best' and what you mean by 'rubber'". The word "best" here suggests that there is a single brand or model of fins that transcends all other brands and models. I don't believe this is the case, however, because everything depends on the expectations of the end-user, namely the snorkeller or diver. The first consideration is how well the fins match the swimmer's foot dimensions, not only length-wise, but also in terms of width and arch height. Then there is the matter of open versus closed heel, not to mention those hybrid fins of the past that came with a heel strap and a heel plate. Moving on to the blade, which size is best? Short, medium and long blades all have their defenders in a variety of aquatic pursuits. What about blade stiffness, vents, splits? The variety is endless, and even if we know what is best for ourselves, who are we to extrapolate from our own choices and impose our selections on everybody?
Then there's the question what we mean by the word "rubber". When I began snorkelling back in the late 1950s, "rubber fins" were certain to be made predominantly from natural rubber, the sap of the rubber tree, a material known chemically as isoprene. When a new stock of fins and masks came into sporting goods stores each Spring back then, you could always tell because of the strong aroma of natural rubber, testifying to its freshness, in the same way as the scent of roasted coffee in a grocer's shop was a testimonial to its quality. By way of contrast, the term "rubber" nowadays has come to mean virtually any kind of "elastomer", whether natural or synthetic.
This said, I've read Lil Dragonfly's other message thread "Great news" to try and establish what the original poster's priorities are in diving equipment manufacturing. One of the priorities is the use of Brazilian natural rubber. I know of only one manufacturer of fins made from Brazilian natural rubber:
Cobra sub. This Brazilian company produces two models of fins, one medium-bladed, the other shorter-bladed:
Spinta
Portuguese description: Pé-de-pato em borracha natural. Ajuste e conforto. Indicado para treinamentos de alto nível. O tamanho do pé indica o tamanho exato no pé-de-pato. Projetada na Itália e considerada por especialistas como uma das melhores nadadeiras de pala média do mundo. Sua matéria prima é a nobre borracha natural da Amazonia, que possui durabilidade e propriedades mecanicas incomparáveis. Perfeita para mergulho em geral, inclusive o autonomo e snorkeling. A preferida de corporações e operadoras de mergulho. Tamanhos: 38-40, 40-42, 42-44, 44-46, 46-48.
English Translation: Fin made from natural rubber. Comfortable fit. Recommended for high-level training. Foot size indicates exact fin size. Designed in Italy and considered by experts to be one of the best medium-bladed fins in the world. Its raw material is noble natural rubber from Amazonia, which has durability and incomparable mechanical properties. Perfect for diving in general, including scuba and snorkelling. A favourite with diving companies and operators. Sizes: 38-40, 40-42, 42-44, 44-46, 46-48.
Rondine
Portuguese description: Nadadeira com pala curta, também frabricada com borracha natural da Amazonia, partilhando, portanto, de todas as virtudes desta refinada matéria prima. Destacase no mergulho em geral e snorkeling. Ideal para natação e hidroginástica. Ampla grade de tamanhos: 30-32, 32-34, 34-36, 36-38, 38-40, 40-42, 42-44, 44-46.
English translation: Fin with a short blade, also manufactured from natural rubber from Amazonia, sharing and bearing all the virtues of that refined raw material. Outstanding for diving in general and snorkelling. Ideal for swimming and aquagym. Wide range of sizes: 30-32, 32-34, 34-36, 36-38, 38-40, 40-42, 42-44, 44-46.
I'll leave it there for the moment. I have no direct experience of either Cobra sub product, but at least they fulfil one of Lil Dragonfly's criteria.