What should I mine?

That is the question.
What you should mine depends on what you want: do you want to get a specific type of mineral for construction purposes (i.e. you want to build ships, equipment, etc), or do you just want to make as much cash as possible? The answer to that question determines what you should do.

Mining for minerals
Different ores result in different minerals, as you hopefully already know. There are a lot of tables on the Internet that can help you figure out what you want, but one of my favourites is Cerlestes. It shows how many minerals you get per 1m3 of mined ore. Looking for isogen? Then click on the Isogen column header to sort the table, and you'll see that your best ore choices are Omber, Kernite, Hedbergite, Gneiss and Hemorphite (in that order). Furthermore, the last column will tell you that you'll only be able to find Omber and Kernite in highsec space. Not bad to know.

Mining for cash
If you just want to make as much cash as possible, that same table will also help you. Click on the 'Price' column header to resort the table. Now you can see what the most expensive ores are, since the price column indicates the value of 1 m3 of ore (it's the little number between brackets), based on recent EVE market prices, including in Jita where we sell.

For instance, as of now (11 August 2010), the top ores are Arkonor, Bistot and Mercoxit. Unsurprisingly, all of those are nulsec ores, meaning you're not very likely to come across them soon.

More interestingly, you'll also see that the top highsec ores are Kernite, Plagioclase and Pyroxeres. In our HQ system, that means you can make the most cash by mining Plagioclase.

Even more interestingly, you'll also see that Plagioclase is ranked significantly above some rarer ores such as Hemorphite and Jaspet. Bet you didn't know that, huh? :-)

Key conclusion: if you want cash, keep an eye on how the market changes. As I'm writing this, Plagioclase is top dog for us since it contains a lot of mexallon, which is expensive right now. By the time you're reading this, that might have changed. Want the most cash? Do your homework first. Or just ask someone in your corp who knows about this.

The pro stuff
So now you know the most important stuff. But suppose you just need isogen for building. Omber's your top choice, right? The text above says so, and the Cerlestes table agrees. But you're dead wrong.

As I'm writing this, the two cheapest ores to mine are Omber (about 69 isk per m3) and Jaspet (about 66 isk per m3). Note that the last one is a lowsec ore, and yet, it is comparatively worthless. But we were talking about Omber.

Suppose you mine 100.000 m3 of Omber. Congrats, that's a lot of hard work. Cerlestes teaches us that that can get you 102.000 units of tritanium, 41.000 units of pyerite, and above all 102.000 units of isogen (yay!). Cash value based on Jita prices for the entire 100.000 m3 is 5.7 million isk. That's not so hot.

Now suppose you mine 100.000 m3 of Plagioclase instead. You also get a bunch of minerals, which are worth about 9.1 million isk, but no isogen. Imagine however that you take that cash to Jita and simply buy isogen on the market. Sales price is around 52 isk per unit right now. You can treat yourself to 150.000 units of isogen (about 50% more than by mining Omber!), and still have over a million isk left to party with.

Of course, all that assumes that you have the resources and time to haul stuff to and from Jita. Still, in high quantities, this type of calculation can make a lot of cash. And high quantities are what a corp is all about...