The more advanced the material, the more money you'll make. Why is this? Because you don't have to MESS with more-processed stuff as much.
If you try, for some reason, to deal in unprocessed stuff like "Aqueous Liquids," you'll be out at the customs office every frickin' day with a hauler, and a load will be worth a few thousand isk at most. Bleah.
Go up one level, and "Water" will be a trip every few days, and it's worth a few hundred isk/unit. We're getting into money.
Process it again and you've got "Coolant" at 10k/ unit, and you're hauling once a week. This is almost practical!
The details can change, but the basic idea applies to everything. I'm working on Robotics, myself, but it's still 1 isk/unit "Heavy Metals" going into 100 isk/unit "Toxic Metals" going into 1000 isk/unit "Mechanical Parts going into 60k/unit Robotics. Same pattern.
One last thought: Think about where you live. What systems are handy to you? What planet types are in those systems? Look at the spreadsheet at https://spreadsheets.google.com/spreadsheet/lv?key=0ArlHEr5t26lndDN3QVhBODJ3YnF5MWVMcUQ0bHdxQXc&f=true&noheader=true&gid=0 . What level-3 commodities can you build by combining only two planets nearby? That's what you build.