They have mixed liquidity. Horribilicus's chart represents the liquidity better. FWIW Open interest is a measure of liquidity because it shows how many contracts are in trade but I find daily volume to be more important. Also, some contracts are more or less difficult than pure liquidity would suggest. I have marked up the chart with yellow (best) to purple (worst) to indicate those contracts that might be chosen for diversity if you were going to trade a trend orientated system. ES for example is poor because, with a daily chart, it doesn't submit easily to most trend orientated strategies (imho). Swiss franc is poor because if you already have CU the two are frequently quite strongly correlated. But these are all just hints. In the end you have to study them and do your own homework
As a measurement of liquidity open interest and volume are practical but don't tell the whole story. Here are just a few other aspects: Open outcry futures that have low open interest but an aggressive floor populations can be more liquid then some others that have a large open interest of hedges or spreads that are seldom moved. In Sugar there is very active two sided trade participation and the contract is very liquid and there is also depth to it. In addition there is some arbitrage with foreign markets. And we should think of depth of market, which is an aspect of liquidity, Liquidity where? At bid/ask or two or 3 ticks away? In some commodities the trade won't let the price get away too far from what they think it should be. So, while they may not be there at bid/ask they are lurking with size somewhere. In currency futures the floor may not be that liquid but there is tremendous depth to it due to arb activity.
Thanks for your help guys - in particular horribilicus and kiwi_trader. I really do appreciate it. All I wanted was a start instead of having to trawl through the each contract data! Kind Regards, Splat