Since most of the cores posted are defensive, I thought I would post an offensive core I've had much success with on the ladder
Metagross @ Lum Berry
Adamant I Clear Body
EV's: 252HP / 220Atk / 36Spd
Meteor Mash/ Zen Headbutt/ Bullet Punch/ Earthquake
Starmie @ Life Orb
Timid I Natural Cure
EV's: 4Def/ 252SpA/ 252Spd
Rapid Spin/ Surf/ Ice Beam/ Thunderbolt
Dragonite @ Choice Band
Adamant I Multiscale
EV's: 8HP/ 248Atk/ 252Spd
ExtremeSpeed/ Fire Punch / Superpower/ Outrage
Dragonite and Metagross have amazing synergy together and work together to blow holes in the opposition. Starmie is used to maximize Dragonite's switch ins and beat the Physical walls that annoy the duo such as Skarmory and Gliscor. Starmie also doubles as a burn absorber and Metagross can take T-Wave and Toxic without an issue so status isn't a great worry for this core either