Im going to post what I consider to be the most affective ubers offensive core. This core maximizes skilled play and long-term planning as opposed to prediction. It is extremely reliable, and it can basically ignore team match-up.
The Core is:
Darkrai@Leftovers, Timid Max Spa, Max Speed
Mewtwo@ Life Orb, Timid Max speed, Max special attack
Arceus Fighting, Timid, max speed, max special attack
This core is operates by sleeping one of their important checks to mewtwo or arceus and then sweeping. Darkrai comes out late game after they revenge kill one of your other offensive threats. If you're against stall you agressively substitute with Darkrai forcing them to let you sleep an important pokemon without relying on risky predictions. I usually support this core with a weather inducer, a stealth rocker, and a ghost for a really basic offensive team that doesn't worry about team match-ups because Darkrai throws any advantages they have out the window.
The simplicity of the ghostlike beast
The purity of what he wants
And where it goes
Always loves you
Always loves with, infrared love