Shuckle, but I have no idea why!! They gave the little boss power split so now he can hit harder and take even more hits. But for some reason my power split+rock slide+rest shuckle always gets crap. And they still keep him in NU, despite being amazing. I may be biased because he's my absolute favorite pokemon, but he can't be that bad... Can someone please explain this to me?
I gotta agree, Power Split Sturdy Shuckle can be quite mean if you play him right. But, you gotta know what you're doing with him because if you're not prepared, his massive defenses won't do much good.
I think the problem with him though is his abysmally low speed and HP, and that the Power Split gimmick takes a turn to set up. Not to mention, the weakness to Stealth Rock. I mean, when he comes in, he's going to lose 25% of his HP, which he doesn't have a lot of. Then, you have to use Power Split, and hope you don't get KO'd. The problem is that since Shuckle's speed is so low, he nearly always goes last (unless you have something like Quick Claw, Trick Room, or your opponent uses a low-priority move like Roar/Whirlwind). Of course, when he survives the first turn and gets the Power Split, you weaken your opponent by a ton (since Shuckle's attack stats are abysmal too), and you can Rest up your HP and you have a created a monster.
The problem is still being able to survive. If your opponent has a Pokemon with higher speed than Shuckle (which is almost every Pokemon in the game, let's face it), and gets STAB on either Water or Rock ... they can easily get the upper hand. Personally, if I was going to make a Shuckle counter, it would be a Rock Head Relicanth. Relicanth's attack stats aren't that high, which means Power Split won't have TOO much of an effect on him, and won't give Shuckle that much of a boost especially if you've already used Power Split once before. He's going to outspeed Shuckle, and can trap him in combat with Whirlpool. Then, can proceed to deal a large chunk of damage to Shuckle using Head Smash. Thing is that Head Smash has 150 power (becoming 275 power with STAB, and 550 because it's super effective) ... with no recoil thanks to Rock Head. Shuckle's great Defense won't do much good here with such low HP, and Power Split isn't going to affect the power of the opponent's move, only the stat used for it. Add into that, Relicanth's 130 Defense, and even Shuckle's Rock Slide probably won't OHKO, even if you used Power Split on something with great Attack and still managed to survive.
But, that's just me and I'm no expert on competitive play. And besides, the new Sturdy mechanic they introduced in Fifth Gen really gave Shuckle some much-needed love. Play him right, and he can be a MEAN little bugger.