Stunfisk is a cool derp looking fish with a unique tiping, nice stats and good movepool, I like it. So, let's make something derp!
Set Name: Status inflicter
Item: Leftovers
Ability: Static/Sand Veil
Nature/EVs: Calm 252 HP, 252 SpDef, 4 Def or Calm 252 HP, 232 Def, 24 SpDef or Bold 252 HP, 252 Def, 4 SpDef
-Scald/Earth Power
-Discarge/Thunder Wave/Earth Power
-Yawn/Earth Power
-Toxic
A nice set, capable of crippling many opponents with the statuses it can inflict. Scald quite powerful and has a 30% chance of burning the opponent. Discharge is STABbed, quite powerful and has a 30% chance of paralyzing the opponent, but Thunder Wave can replace it as it always paralyzes. Yawn forces switchs and can also put the opponent to sleep. Toxic badly poisons the opponent. Earth Power can be used to get a nice stab. It also works well with toxic since it hits poison and steel types immune to it. The move to get rid of will most likely be Yawn, since it doesn't work on pokemon already statused and you will be spreading many statuses. I suggest three possible EV spreads. The first is completely specially defensive since it's your best stat. The second gives you mixed defences with 422 HP, 262 Def and 264 SpDef. Probably the best spread for mixed defences since your SpDef exactly gets a jump stat and makes you defences around the same, with SpDef slightly higher than Def, so that Download abusers don't get the threatening +1 SpAtk. The last spread is completely phisically defensive if you want to abuse your rock and steel resintances, the only problem is that horrible ground weakness. For the nature both Static and Sand Veil are good. Static may be redundant sometimes. Remember to chech whether your moves are legal with Sand Veil.
Other Options
Protect stalls toxic, burn and sandstorm damage, helps getting more leftovers (the only form of recovery) and works well with Yawn, so is very nice. I suggest not to replace Toxic with Sludge Bomb for the poisoning chance. Normal poison is just an inferior burn, so just use Scald. Not to mention that you can burn steel and poison types, while fire types immune to burn won't enjoy a scald and water veil pokemon won't enjoy a discharge. Replace Discharge with Thunder if you are playing a rain team: is a free +50% power.
Partners
Well, you have four weaknesses: water, ice, grass, ground. A grass type resists three of them, while you can take Flying and... uh, poison attack aimed at it... also something to take out grass types as they resist everything you pack. Flying and Fire types are particulary good, and have good sinergy with you.
Counters
Magic Guard don't mind to much being statuced, expecially Reuniclus, who will just calm mind up and annihilate you. Clerics (expecially Blissey/Chansey and Clefable) make you cry a lot. Grass types resist your offensive moves and kill you with a grass type move. Offensive ground type don't like Scald but are immune to paralisys and hit you very hard with a ground move.
Set Name: Rest talker
Item: Leftovers
Ability: Static/Sand Veil
Nature/EVs: Calm 252 HP, 252 SpDef, 4 Def or Calm 252 HP, 232 Def, 24 SpDef or Bold 252 HP, 252 Def, 4 SpDef
-Earth Power/Scald
-Discarge/Scald
-Rest
-Sleep Talk
One of the few 5th gen pokemon that get Sleep Talk, so let's put it into use. Earth Power and Dicharge get good coverage, STAB and paralisys support. Scald can replace one of the two for burn support. Same EV spread and nature as before.
Other Options
You will want only theese moves. Thunder over discharge in the rain obviously.
Partners & Counters
Same stuff listed before.
Set Name: Put sunglasses on this derp eyes!
Item: Choice Specs
Ability: Static/Sand Veil
Nature/EVs: Modest 252 HP, 252 SpAtk, 4 SpDef
-Earth Power
-Thunderbolt | Thunder
-Scald/Surf
-Hidden Power [Fire | Ice]
Stunfisk has amazing defences, a doable special attack, a very good offensive typing, one immunity and a resistance to Stealth Rock, so it can use Choice Specs very effectively. Earth Power and Thunderbolt are your main moves, powerful, accurate and get very good coverage together. Your water move of choice is used to hit ground types immune to electric. Scald vs Surf is burn support vs power. Personally, I'd use Scald, as the occasional burn damage will compensate for the lack of power. The last move covers grass types as they resist everything else. Pick either HP [Fire] or HP [Ice]. Fire is recomended for Ferrothorn. You may want to use Stunfisk in a rain team, as it is immune to electric and can abuse 100% accurate Thunder and pseudo-STAB on his water move. If you do so, pick HP [Ice]. Obvious EV spread: max HP to maximize both your defences and max SpAtk to pack as much punch as possible.
Other Options
Probably these are the only moves you will want. Maybe Sludge Bomb for grass types, poisoning and nice power, but grass/steel, grass/poison, grass/rock and grass/ground laugh at it, probably Hidden Power is better.
Partners & Counters
Same stuff listed before.
Set Name: Banded fish
Item: Choice Band
Ability: Static/Sand Veil
Nature/EVs: Adamant 252 HP, 252 Atk, 4 SpDef
-Earthquake
-Spark
-Stone Edge
-Revenge/Payback
Not as good as the Specs set due to less power and worse moves, but has the surprise factor. Pretty straighforward. It really only has five usable phisical moves. You may want to use Brave Nature and 0 IVs in the speed to make Revenge/Payback go last and double the power.
Set Name: Cursing you with his derp eyes
Item: Leftovers
Ability: Static/Sand Veil
Nature/EVs: Adamant 252 Atk, 252 SpDef, 4 Def
-Curse
-Earthquake
-Stone Edge/Rock Slide
-Pain Split
Curse is your only boosting move, and you have a good special defence, so probably it's good. It boosts your Atk and Def, while you don't care at all of the speed drop. Earthquake for STAB, a rock move for great ground/rock offensive combination. Stone Edge is more powerful, but Rock Slide has better accuracy and better PP. Pain Split is used to recover health while you boost up. The EV spread is very important: Adamant and 252 EVs in the Atk are pretty much necessary because you attack stat is quite low and this heavy investment will allow you to need less curses to sweep and will increase you chances of doing some nice damage. As you're using Pain Split I recomend not to invest in the HP stat, as it is already huge. 252 EVs in SpDef maximaze your special defence more than 252 HP and allow you to fully abuse Pain Split. After one curse your stats are 359 HP, 381 Atk, 307 Def, 178 SpAtk, 297 SpDef, 66 Spe. After two curses the stats are 359 HP, 508 Atk, 410 Def, 178 SpAtk, 297 SpDef, 50 Spe.
Other Options
Too bad Spark is illegal with Curse... you can opt for Substitute instead of Pain Split. In this case, the EV spread can be switched to 252 HP, 252 Atk, 4 SpDef.
Partners & Counters
Same stuff listed before.