relativity
Well, I thought for a while that PIAA bulbs were the best for everything, but I've learned a lot about lights, colors, lenses, etc. since then, and I've come to the conclusion that headlamp bulbs, and vehicle lighting in general, is one of those things that there is no one "best" of...theres different types and kids and brands, and they have different functions, and are good and bad...in different ways.
I'd say a PIAA superwhite or similar "white" bulb is good for general driving, but like Bill said, they disappear in rain or wet road conditions- due to the color it seems. The cheapo "blue" bulbs are even worse. Other than that, "white" bulbs have a nice soft light and are great- even in snow.
I've decided that for my purposes, the PIAAs with a set of yellow fog lights gives the best of both worlds. I think if you have a set of fog lamps with regular bulbs or "ion" bulbs/lenses, you get the benefit of the white headlamp bulbs, but can use the fogs when your white or blue beam disappears in the rain.
I also feel strongly that for "all-around" use in both nice weather and when its raining, just regular ol' bulbs are whats called for. Your average driver needs to see in both clear and nasty conditions, and probably doesnt want to pay $80 for bulbs anyway.