Which 'open' to consider?

For trading system development, I am currently analyzing the EOD data of various markets. However I am not sure which market 'open' should I choose - the actual market open as stated by many market data providers or the futures' open?

The market open used by data provides like Yahoo/Google is quite near to the previous close - and doesn't take into account the gaps.

Which one is more effective in analysis? What do you use?

Could you be a bit more vague?

Forex, Futures, Stocks, Bonds????

Basing TA on the Open value is fraught with risk. For example, different data providers quote the FTSE100 Open as the previous day's Close, the value at 08:00am or the value at 08:02am. Overnight gaps mean such discrepancies can be significant. The FTSE is an index that actually has a closed session overnight, i.e. it is not being continuously re-calculated. But spreadbet markets derived from the FTSE (as well as Forex etc.) are open throughout the 24 hours, so Open in that context has much less meaning.

My advice is pick any (toss a coin to decide if you must) then get stuck into the reseach ASAP and get your feet wet

Thanks for all the replies.

Yes it has been my experience - data for Open is quite erratic between the data providers.
Can any one please share a good source of reliable data for futures' [DJIA/FTSE]?