[Monsieur]

Originally Posted by MadMax

You don't necessarily want to optimize for the most commonly searched term, as that's usually the most competetive

That's what I meant when writing

Originally Posted by Monsieur

To the initial question:
You should optimize for the one that
a) gets the most searches or
b) has the better search/result ratio
depending on your seo skills.

If you're sure you can achieve top positions for every keyword you target, then of course you should go for option a). But if you're just a small webmaster, you should go with option b) just because that's how you'ld end with better chances to get good listings ;-)

I just thought that that's pretty obvious because not a single webmaster who just builds galleries and free sites will ever achieve a top20 position for a keyword like "sex" ;-)
Always choose keywords which aren't searched for that often. You might get less traffic than with another keyword but your traffic will probably be of a better quality and convert better!