General Exhaustion:
* Tired
* Fatigued
* Weary
* Spent
* Drained
* Worn out
* Depleted
* Burned out
* Knocked out
More Intense Exhaustion:
* Prostrated
* Enervated
* Exhausted to the bone
* Completely wiped out
Exhaustion from Specific Activities:
* Run-down (from illness)
* Beat (from exercise)
* Used up (from work)
Figurative Exhaustion:
* Stale (for ideas or feelings)
* Depleted (for resources)
* Tapped out (for money or resources)
The best synonym for "exhausted" depends on the specific situation and the desired level of intensity.