Since I learned about natures (what must have been five years or more), I was used to prefer offensive natures. To be more specific, Modest and Adamant have always been my favorite and is probably the most required natures as I could notice in the RMT threads. However, lately, I am trying to effort a little more on Speed. Since having around +30 points on Attack or Special Attack is such a nice thing, if your Pokemon's Speed is higher than foe's, then it attacks twice as long as foe will attack once (at least most of the times it'll be like this). Unless the move is super effective or from a sweeper into a glass 'mon, then thanks to Speed your Pokemon allowed to attack twice, or even three times in rare cases.
So, post your opinion whether competitively you prefer offensive natures (the ones that raise Attack or Special Attack) or Speed natures (every that rasies Speed).