Which is the best example of chemical weathering?

Rusting iron because of oxidation is the best example of chemical weathering. Oxidation occurs where there is ready access to the atmosphere and oxygenated waters. The minerals most commonly involved in this process are iron, manganese, sulphur etc. The rust is formed from a redox reaction between oxygen and iron in an environment containing water (such as air containing high levels of moisture). 

Further Reading:

• Chemical Weathering 
• Weathering