Mega Poliwrath
Type: Water/Fighting -> Water/Fighting
Ability: Damp/Water Absorb/Swift Swim -> Sap Sipper
New Moves: Drain Punch
HP: 90 -> 90
Atk: 95 -> 125 (+30)
Def: 95 -> 125 (+30)
SpA: 70 -> 100 (+30)
SpD: 90 -> 120 (+30)
Spe: 70 -> 50 (-20)
Flavor Concept: Poliwrath hits the weights and goes the Mega Swampert route of crazy new bulk.
Competitive Concept: First off, it's a crime that Poliwrath, one of the first bulky fighting types out there, doesn't get Drain Punch. Let's nip that shit in the bud right there.
Second, Poliwrath is one of those jack of all stats, master of none mons, so I've given it simple +30 to each of its stats except Speed, which is nerfed. This makes it sit pretty at 90/125/120 bulk, and great mixed 125/100 attack stats. While it can run all-physical, with Drain Punch, Waterfall, and whatever coverage or utility you want (Substitute, Bulk Up, Encore, the tadpole's got some range), it can also run mixed to use that sweet, sweet Scald. A pure defensive investment with Scald and Circle Throw also works behind Poliwrath's new defenses. Either way, Poli can afford to run a speed-lowering nature to keep its mixed movepool hitting hard.
Sap Sipper not only rids Poliwrath of a weakness, but boosts its attack even further; moreover, it's rendered immune to many powder moves and Leech Seed. Water Absorb still allows for some great switch-ins pre-mega, but a grass immunity that boosts attack is just perfect for Poli, leaving it weak only to Electric, Fairy, Flying, and Psychic; if that sounds like a lot, bear in mind it ignores Grass and resists Fire, Water, Ice, Rock, Bug, Dark, and Steel. Scizor? Heatran? Tyranitar? Just a few of the mons walled by this monster. Water/Fighting is terrific defensive typing, and Mega Poliwrath uses it to its fullest.