* Overall best-selling model: This would be the DeVille, which was produced for 55 years (1949-2005) and sold millions of units.
* Best-selling model in a specific period: For example, the Escalade has been the best-selling Cadillac in recent years, due to its popularity in the SUV market.
* Best-selling model in a specific region: The answer might vary depending on the market.
To give you a more accurate answer, please clarify what you're looking for!