In large Splunk environments, the 'stats' command is the most efficient for grouping events by fields. It is highly optimized for performance and allows for aggregation and statistical calculations on the data. The 'join' command is less efficient for large datasets as it can be resource-intensive, 'streamstats' is used for calculating running totals and cumulative statistics which is not suitable for basic grouping, and 'transaction' is used to group events into transactions, but it is more resource-intensive and slower compared to 'stats'.