Dialga, Lucario, Metagross, Skarmory, Magnezone, Scizor
Dialga: An Uber through and through. It should know:
Lucario: With the ability to do massive damage, this guy has helped shape the current metagame. It should know:
Skarmory: Use the moveset I gave in your Flying question.
Metagross: One of the most powerful Pokemon in the game. It should know:
Magnezone: Another Pokemon that has shaped the metagame thanks to its anti-steel powers. Great to deal with other STeel teams while also providing an Electric type. Should know:
Scizor: Another Pokemon that has greatly shaped the metagame, like most steel types have. It should know: