I found the stock EQ clipped really badly when I adjusted it, but the 5 band one through rockbox is much better. I usually use it to boost the 20-40Hz frequencies to accentuate bass/drum lines and reduce a wide range of the treble slightly to turn down the harshness of the highs on my ER4Ps.
Since I don't want to be a smartass, I'll say treble booster. I only do that once in a blue moon though when I listen to a rock song that really needs it. My battery only lasts a good 3 hours (I've been meaning to replace it ever since I've gotten it), so EQ would worsen it.
I agree with the comments that "off" or "flat" is the best. But if you're going to use one of the settings, rock and R&B are the only ones I can stand. I don't know who designed the iPod EQ but whatever they were paid was too much. Ironically, the iPod's onboard EQ is not equal to the EQ that comes with iTunes. The latter is not the world's greatest EQ either, but at least with it, I can manually set it and get some interesting effects.