It's near impossible for anyone to give you any definitive answers without knowing your server's economy (do NOT post your server name). On some servers, Obsidium and Elementium ore isn't being farmed by many so would be great, on others, it wouldn't get you much. The same goes for Cata herbs...
You need to research your AH or TUJ (theunderminejournal.com) to figure out what sells well and for decent coin on your server and then get that... on many servers, Northrend and Outland ores and/or herbs are in short supply so prices are high... but the demand for them isn't really high. So... if they are selling high, get some of each...
But in short, do your own research and you can take it to the bank...