Holy crap, even Scarf Electrode doesn't outrun a few rain sweepers.
Jolly Electrode hits, if my grasp of the base stat formula is correct, 449 or 450 speed. That's either 673 or 675 when scarfed. I think +Speed Lumineon hits 685 in the rain. Floatzel hits 719 when neutrally natured and 790 with a Jolly nature. Anything below Lumineon should be easy pickings. (Lumineon too, if the trainer gets cocky and doesn't fully invest in speed EVs.
So between Expert Belt Ninjask and Scarf Electrode, you have a pair of serious countersweepers.
And that Ninjask set I promised to outrun Sawsbuck: (and now certain Jumpluff!)
Ninjask@Expert Belt
Nature:Jolly
Ability: Speed Boost
EVs: 252 Atk / 4 Def / 252 Spe
-X-Scissor
-Aerial Ace
-U-Turn
-Protect
493 or 494 speed to start with. Huh. It hits either 739 or 741 after one turn of speed boost. Using 300 as Jumpluff's base, this set will outrun neutral natured Jumpluff by a fairly wide margin. Jumpluff apparently hits 699 with a neutral nature or either 768 or 769 with a +speed nature. There's no point in investing more EVs to catch +speed Whimsicott or Cottonee, but this spread will get past either with a neutral nature. (Or little speed investment, I suppose.) Max Speed Sawsbuck actually only hits 702 or 703, so this Ninjask goes for 704 after a Speed Boost.
Ninjask@Expert Belt
Nature:Jolly
Ability: Speed Boost
EVs: 252 Atk / 92 Def / 164 Spe
-X-Scissor
-Aerial Ace
-U-Turn
-Protect
As far as I know, this set outruns everything that Ninjask can possibly outrun after a single speed boost. Anything faster, and you have to either try for another Speed Boost or already have one. (Outrunning and KO'ing a Sawsbuck would let you slaughter any Jumpluff, too!) That defense will actually let Ninjask survive any Aqua Jet once, even in rain. Mach Punch is obviously not very effective. Weavile/Sneasel/Mamoswine's ice shard is something to watch out for, but it can tank any weaker ice shard once. Vacuum Wave will do nothing anyway, so it's best to focus on physical defense. Sucker Punch from anything with base 92 attack or more will slaughter Ninjask, so watch out. (Haven't done the calcs for all the Shadow Sneak, Bullet Punch, Quick Attack, or ExtremeSpeed yet.)
There is speed creep, and then there is Tier Shift.
NINJA EDIT: I knew something was slightly off there. Redoing speed calcs. Everything but Ninjask and Electrode was off by 100 or 110 depending on nature, is all.
EDIT EDIT: If you're using Ninjask against rain teams, you can go for a 252 Atk/ 60 Def / 176 Spe spread to outrun Luvdisc. You know, if anyone were to use it.