The most buoyant rubber is probably the new Yamamoto AeroDome 5mm with the air cells in it (it's actually a sandwich of three layers). The most buoyant wetsuit would be made entirely from this...and it would suck. Performance in a wetsuit is a combination of buoyancy, flexibility, lowered surface drag, and body positioning. This is why suits use different types and thicknesses of material throughout...manufacturers are trying for the optimal balance of the above factors (neglecting the cost factor for a second).
Sadly, there is no easy way to determine what the fastest suit is (and it varries by individual). Every manufacturer's top-of-the-line suit is the fastest wetsuit on the planet, just ask 'em. The best advice is to go with the suit that is the best fit to your body and budget.
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