Choice Scarf Flygon
Choice Band Scizor
This is probably the defining core of late gen 4 bulky offense, combining incredible scouting and revenge killing with great type coverage and resistances. Water types and stall in general are the only things which trouble this core. Combining this core with things like Heatran, Gyarados and Celebi is one of the most common team structures around.
I should also note, there are just individual
pokemon which lend themselves to cores, due to their incredible typing etc. Throwing some of these together almost always make solid cores.
These are so standard because they work so well together.
There's some other more unusual cores, like Magnezone/Dragonite which resists everything in the game and has good offensive synergy too.