Wow... guys, really? Has anybody played gen 6? No longer a grind. I beat y version in VI days. It took me 23 hours of game play. I will admit it isn't the best generation. Gen IV is the best. Don't argue, just accept. Anyway, the most powerful, according to Pokedex entries, is Magcargo. It's hotter than the surface of the sun. Anything within a several mile radius of one would instantly combust. The most powerful in game is Mega Mewtwo Y, with its insane Sp. Att, awesome Spd, and viable defenses. The best according to what I believe, would have to be, actually, Groudon. This is purely because of one thing: it would have dried up the seas if not for Kyogre. Thus, the intensity of the sun must have increased vastly. Groudon is powerful enough to make the seas dry up, and, thus, could pretty much wipe out everything in existence.
Just saying.