As The Music Man correctly says the Lady Blunt Strad is the most expensive ever sold. However, this price is believed to be rather inflated because it was sold at a charity auction to raise money for the Japanese Tsunami relief fund. More typical values are $1.5 - 3.5 million.
However, which of his violins is 'the best' is a much more subjective matter and one of personal preference. Even such a highly esteemed maker as Stradivarius made bad instruments as well as good - after all he was someone who would experiment in search of perfection rather than just reproduce violins to a set formulae. That is always going to result in a variation in quality.
Further, it is worth noting that amongst 'violin people' the Guanarius del Gesu violins are held in as high esteem as those of Stradivarius and command equally high prices. I am still not convinced as to the reason why the Strad has become so widely known whereas Guanarius is a name little known outside the inner circle of the violin fraternity. It has been suggested that it is because the biggest names in violin playing have all played Strads rather than del Gesu violins, but this simply is not the case. For example, perhaps the most widely known living violinist at this time, Nigel Kennedy, has a Guanarius violin.
You can see the somewhat lesser violins I have in my collection at http://www.virtuosiviolins.com