For most active day traders the eMini SP 500 is the market of choice. There are quite a few reasons for this.

Volume
First and probably the single most important reason is volume. The ES does an average of 3,000,000 million contracts per trading day making it one of the highest volume markets.

Liquidity
Liquidity is a direct function of trading volumes, so a high volume market also offers higher liquidity. Liquidity more precisely refers to the amount of volume at each price level. With higher liquidity a trade is more likely to get filled at the desired price. Whereas with low liquidity markets, there is a greater chance for slippage (getting filled at a price other than the desired price)

Point Value
The point value for the eMini SP is $50 per point and it trades in quarter point increments. This means that the minimum tick value, smallest possible price move, has a dollar value of $12.50

A market that trades in smaller price or tick increments tends to be preferable for smaller traders as it allows them to trade multiple contracts and scale out of trades for partial profits

Trading Hours
Regular session or what can also be referred to as Pit Session hours are from 9:30 am to 4:15 pm EST.
Globex trading, also known as the extended trading session, is 23.5 hours, with a daily maintenance shutdown 5:30 p.m. - 6:00 p.m.

Low Broker Required Margins
Due to the high volume high liquidity and consistent volatility the eMini SP has an additional advantage in that many futures brokers offer very low margin requirements to trade this market.

Margin refers to the amount of money in your trading account that is required by your broker to trade 1 contract. Without using any margin, a single contract would require $50 x the current value of the index. If the market is trading at 1,300 it would therefore be 1,300 x $50 = $65,000.

Many brokers offer margin as low as $500 to $1,000, or .75% of the value of the contract.

For more updates from Greg, be sure to visit his blog at TheTradingZone.com.