For your example (a simple string), System.debug(value);
is sufficient. If the value you want to debug is more complex, such as an array with many entries, an SObject record with long field values, or an apex class with many properties, System.debug()
often truncates individual field/property values and/or array members. If you run in this problem, a useful workaround is
System.debug(JSON.serialize(value));
JSON-serialized output is not truncated in the log (unless the whole log is so large it it truncated), although it may be truncated in Developer Console; viewing the raw log or downloading the log will let you see the full value.
Note that this can rapidly increase your log size for very large objects/arrays, but when you just need to see the contents of a variable/expression without truncation, it is a useful option. For large outputs, a JSON formatter may be useful; there are many online.