I agree that mode function is most likely what @HeinzWaescher was looking for.
Because it is a stats function, you can also use it with eventstats to keep the events.
| eventstats mode(country_code) by user_id
For the example initially provided, the top command does not appear any better than the stats mode() command provided by @p3hndrx. But if you the top N values (N>=2), there is no function like values() that returns them sorted by frequency of occurrence.