Anything that has volume is liquid. WTI CL has anywhere from a 3-6 cent spread depending how volatile the underlying is, and you can get filled 50-100 lots without a problem. NG has a very tight spread almost always, and you'll never get filled except at th bid/offer. HO has a ridiculously wide spread.Quote from trading spaces:
What are the liquid commodity options on the screen?
Grains have a wider but reasonable spread, not too hard between the ask/bid... and best of all is quoted even overnight (altho spread grows). Equities also have reasonable tight spreads, especially ES.
In the NYBOT softs, non-serial month options are usually quoted but with a fat spread. Only place IMO where option pits still very relevant.