Shoulder

'Cold shoulder' indeed.

The shoulder is one of the most vital components of a Transformer's body.

It is unknown why the shoulder is such an Achilles heel of Cybertronians. However, it is clearly the most easily damaged location on their chassis, as many instances of minor damage have led to grave injury and even death.

Such instances include:

• In a battle on Earth, Ironhide was shot in his shoulder by Megatron. Seeing this injury, Optimus Prime immediately called off the attack to get Ironhide medical attention.
• Years later, the incredibly strong Autobot Brawn was killed in a single blow from a wound to the shoulder. The Transformers: The Movie
• Catilla was killed by a blow to the shoulder from Bludgeon's sword. A Savage Place!
• Megatron once decided to retreat after sustaining severe shoulder damage, although this may have been from worry that another blow could pierce his armor elsewhere. Haven

There are very likely many more examples of shoulder wounds and their fatal nature. The only theory is that many Transformers store their spark in their shoulders.