The most suitable approach for designing a load test to validate performance across different regions in a global Azure Databricks solution is to simulate geo-distributed data sources with Azure Event Hubs in different regions, direct data to a single Databricks workspace, and monitor latency and throughput. This method allows for the simulation of real-world scenarios where data is ingested from various locations, tests the solution's efficiency in processing data from multiple regions, and helps identify potential performance issues by monitoring key metrics such as latency and throughput.