Overview (in my own style this time, don't worry, Webmaster)
Well, Kingdra is odd in a good way. These stats seriously need a boost, and luckily, Kingdra gets Dragon Dance. In Gen II it wasn't outclassed either, it was entirely different from the other Dragons. Like a warstory's title I've read lately, Kingdra always said: "I'm not going down". In Gen III it wasn't smashed with Salamence's arrival, or outclassed by Flygon. I mean, it faced trouble for other reasons, but it didn't go down, and in Gen IV, it got all it needed to rule OU. Palkia wasn't trouble as it was sent to Ubers. Gen V... What's coming now? Well, it was dropped into UU, but it's not going down, still. It didn't get an useful Hidden Ability, but it doesn't matter for it. It has always got one good advantage: Only weak to Dragon, and Dragons themselves risk to receive a boosted Outrage or a Dragon Pulse or Draco Meteor in their faces... In Gen VI the Fairy-type might ruin this advantage, so... What's next? Don't give up, Kingdra!
Adamant or Jolly
Leftovers/Expert Belt/Muscle Band/Lum Berry
252 Atk/252 Spe/4... HP, Def or SDef???
Got my pun? Kingdra... gon Dance? OK, let's go. Well... Expert Belt over Life Orb, and there's the reason. Often I see sets with Substitute, Life Orb and no Recovery or Magic Guard or at least Sheer Force... Fatal Error. This is smashing yourself. Muscle Band is also usable if you don't think you'll hit much super-effectively. Or Leftovers, to recover after setting up a Substitute. Lum Berry, however, allows a second Outraging rampage. Anyway, use Substitute on something that can't hurt you too much, Dragon Dance once or twice depending on what you're facing and attack with your fierce STAB moves. Switching might be forced by an incoming Empoleon, but if you have obtained enough boosts (+3 for offensives, I guess, and +5 for defensives???) it will eventually fall as well. Nothing else in UU resists your combination, so Kingdra, when used correctly, can sweep entire teams.
Other usable sets
A set more suited for Rain can be used with Swift Swim. Self-Supporting Rain is usable as well.
Mixed Dragon Dance started to show off in the competitive environment, and it can work in Kingdra as well.
Agility is usable, but is very outclassed by Dragon Dance due to the power loss.
Choice is really helpful, if you were looking for an instant boost.
Sniper VS Swift Swim
If you're using it in rain S-S, if not, Sniper. Also, if Kingdra runs Rain Dance for self-support (and not in a rain team), run S-S but forget about Damp Rock and use Life Orb or something like that. DON'T USE DAMP.
Hahaha. Only dragons hit it supereffectively, and risk fainting from Kingdra's own Dragon power. Scarf Flygon will outfly as long as Kingdra doesn't reach +2, case in which Kingdra outswims even without Speed boosting nature. So, hitting hard and fast with regularly effective attacks, even more so if boosted, are the "safe" route, but you're still risking to faint. If it comes into play, better hit it from the first turn. Priority really hurts Kingdra, too. Empoleon will tank it if it hasn't boosted enough. And remove rain if it is Kingdra's power source to shut it down. And smash it after confusing from Outrage. You need smart play rather than pure power to get rid of Kingdra.