A few thoughts:
1. You can ignore the ICE grain contracts. They are CME lookalikes with very little volume.
2. You can ignore the CME softs. They are ICE lookalikes with very little volume.
3. The CME and ICE both have active energy contracts so it's not as clear cut. That's just FYI in case you look at energy trading in the future.
4. Using Interactive Brokers for agricultural contracts, choose the "ECBOT" contracts that start with a Z.
- ZC corn
- ZW wheat (soft red winter). I believe the recently acquired Kansas City Board of Trade wheat contract (hard red winter) is designated "KE" for the e-version.
- ZS soybeans
- ZL soyoil
- ZM soymeal
5. The ECBOT contracts are the ones that trade from 7P CT to 1:15 CT the following day, they have 90-95% of the volume and execution costs are cheaper than traditional pit trades. If you were trading very large and complex option orders, you *might* consider pit trades (I think they are maintaining about a 50% market share), but that market will probably go all-electronic someday, too.
6. As for volume and open interest, you'll usually find the most activity in the first or second month. On my summary quote page I always display at least the first three months and the detailed page has more. Even if you're not a spread trader, it's helpful to keep an eye on what's going on in different months - volume, open interest and price relationship.