My favorite Octillery set:
@Leftovers
Inconsistent
Timid
244HP / 12SpD / 252Spe
-Substitute
-Protect
-Toxic
-Surf / Flamethrower
Pretty much stall for boosts for as long as possible, and use toxic and flamethrower accordingly. It has singlehandedly ripped apart entire teams, with a little luck. Really the only things that caused trouble were Heatran (who were reluctant to be switched in for fear of a water attack), RestTalkers (especially Suicune), Refresh users (they exist), Starmie in the rain with Thunder, CM Rankurusu, and possibly Roobushin (with awful luck or bad prediction). Given twenty or so turns of set-up (made easier with evasion boosts), literally everything else falls to the mighty Octillery!
Edit: Oh, and Manaphy in the rain. However, Octillery does have one fierce Struggle attack if it ever comes to that, considering it will almost certainly be at +6 Atk / +6 Spe! At least it can go out in a blaze of glory, which is truly a wonder to behold.
2nd Edit: Added in Surf as the primary option, because after playtesting, it is generally the superior option. Missing super-effective hits on Nattorei, Skarmory, and Grass-types is well worth the increase in power against rain and sand teams.