To be a counter you need to be able to switch into ANY commonly used move, and either OHKO it before it hits you, or take a hit and OHKO it (or beat it before it beats you).
Aggron, therefore, is not a counter. It can't switch into any offensive move as it breaks sturdy, it has to perfectly predict a dragon dance. Even then, Salamence can just switch or use a weaker move to break sturdy without taking too much from Metal Burst.
Greninja, likewise, cannot switch in at all. Like, /at all/. Any move will do colossal damage, and dragon dance will cause Mence to outspeed.
The only thing that's coming close to countering is like Bronzong, but even then it doesn't exactly do much and is a rare site in OU.