That's a really hard question to answer. There have been many, many variants of Reynolds tubing and each of those has had sub-variants. There's no 'best' tubing as it depends entirely on what you want to do with it.
As an example, of the many variants of 531, just a short off the top of my head list includes 531c (road racing), 531ST (touring), 531 All Terrain (obvious), 531 ATB (heavier duty off road), 531 Pro (lighter version before 653 arrived), 531SL (super light) 531 tandem, the list goes on.
Generally the higher the number the better the strength to weight ratio, but nevertheless you wouldn't want an MTB made from 753 or indeed a road bike made from 753 if you're over about 12 stone. Higher end modern bikes are made from different materials for different tubes - you might find a bike with 853 main tubes and 631 or 525 stays to get the type or ride the builder wants.
They had a close association with Raleigh as for a long time they were both owned by the same private equity firm, TI or Tube Investments. The last big thing they worked on together was the steel Dynatech & M-Trax frames. Reynolds supplied a custom version of their tubesets (2055 = 501, 2060 = 531, 2070 = 653) that were bonded into lugs. The special tubesets came about due to the bonding method. In normal brazing the tube becomes weakened by the heat and so it's made stronger at the tube ends. Because these were being bonded there was no need to strengthen the tube and so the frame could be theoretically lighter. I say theoretically as they didn't actually seem to be lighter.
Both Raleigh and Reynolds were sold off - Raleigh to the Derby International group and Reynolds to a US steel company, although I think there's been some kind of management buy out shenanigans and they're once more UK owned.
