It's also a bit more complicated than just total liquidity, as different markets have different kinds of order flow. For example, in pit-traded futures options, you can usually get in and out of small size in bean options with a one tick spread ($6.25) even in deep OTM stuff, because there are a couple of arb firms that do the other side of any small stuff, regardless of strike. In the Ten Year option pit, there is MUCH higher volume, but it is concentrated in very large orders (1000+, often much larger) in a relatively few strikes, so if you are doing anything else, you are going to get a bit screwed. Futures pits are much the same, in terms of being idiosyncratic. There are lots of people in the ED pit who spread 40 different futures contracts with a good bid & ask, and other pits where if you aren't trading front month, you will get killed, even if the overall total volume & open interest is high. In the E's, just get on the screen and watch and get a feel for bid & ask & size and depth of what you think you want to trade. It's right there in front of you. It really pays to take some time and get to know both the "numbers" as well as the specific characteristics of what you are trading.
Good Trading
Jessie