Quote from sle:
We have gone through the whole list. The core EOD data is more or less the same for all vendors - we have actually made a mistake of thinking otherwise and found out that bad days in historicaloptiondata are almost the same for LiveVol. As far as implied volatility calculations go, Optionmetrics is probably the best one, followed by the Livevol and iVolatility. However, neither are using proper industry strandards and volatilities for some indices and some single stocks are unusable. So we opted to buy EOD data from historicaloptiondata and intraday quotes and trades from TickData.com and re-write all of the calculations on our own.
In terms of pricing, LiveVol is the most expensive of all, Optionmetrics is second and iVolatility is third. There are one or two vendors that provide options data for 650-750 for the whole set (historicaloptiondata is one of them), the data is the same quality, it's the post-processing and treatment of the corporate actions that differs.