Another extremely powerful Pokemon now that Diamond and Pearl has separated Special and Physical attacks for all types is Gengar. STAB Shadow Ball will kill anything in 1 hit without godly special defense, Psychic and Thunderbolt cover remaining types very well. Hypnosis is a decent backup (though a gamble at only 60% acc.).
If you want RAW physical power, nothing beats Choice Band Tyranitar. Choice Band doubles your attack but limits you to only one move (until you switch out). With enough speed EVs Tyranitar will oustpeed all the medium speed Pokemon and has enough defense to avoid an OHKO from anything but a powerful fighting move, but with Crunch, Earthquake, Stone Edge, and Pursuit will OHKO pretty much everything in the game with ease. That said, he probably won't live forever, and the fact that he brews up sandstorms can be trouble unless you have a team built around sandstorm (in which case it's a great strategy).
