
http://www.serebii.net/pokedex-bw/055.shtml
Abilities: Damp / Cloud Nine / Swift Swim
Base Stats: 80 HP / 82 Atk / 78 Def / 95 SpA / 80 SpD / 85 Spe
Golduck, a wonderful Pokemon to counter or revenge-kill Pokemon that attempt to use sun... It's almost like a UU Politoed/Rayquaza mixbreed!... emphasis on the UU.
Sample sets:
Golduck @ Choice Specs
Ability: Cloud Nine
Nature: Timid
EVs: 252 SpA / 4 SpD / 252 Spe
-Surf/Hydro Pump
-Ice Beam
-Focus Blast
-HP-Electric / HP-Grass / Psyshock
Using Choice Specs to boost Golduck's average Special Attack stat to a respectable level, Golduck can smack down lots of common Pokemon that it outspeeds. The top sun threats Golduck can outspeed, however may outspeed it, even without Chlorophyll boosts. Though Golduck can definitely take out Vulpix, -1 Victini, and Victreebel. With prediction as well, you can take out threats as they come in (example: predicting a Sawsbuck switch-in and Ice Beaming it). You may drop Golduck's speed to outspeed standard Victreebel (Modest or Timid) and add more to Golduck's HP for a little extra bulk.
Golduck @ Choice Scarf
Ability: Cloud Nine
Nature: Modest/Timid
EVs: 252 SpA / 4 SpD / 252 Spe
-Hydro Pump/Surf
-Ice Beam
-Focus Blast
-HP-Electric / HP-Grass / Psyshock
This allows Golduck to come in and do what it's supposed to: revenge kill nasty sun Pokemon (namely Jolly Sawsbuck and the occasional Jumpluff and Timid Lilligant), it could come in on predicted fire attacks as well, but can't take too much residual damage. You'll be lacking the power of the Choice Specs set, but Golduck can still clean up sun teams decently.
Other possible sets: SubPunch (perhaps with Disable), Physical Choice (meh attack, but it has a decent physical movepool), SubCM (meh since Chansey still breaks your subs), Icy Wind (meh, but it allows you to outspeed whatever comes in outside of choice scarfers)
I haven't run any calcs yet so feel free to run them if you'd like! I'll probably add them later.