What's considered the best table sorting indicator?

I've got a 'classic' table which has table heads with field names in it:

There are a lot of ways to display an sorting indicator in a table:

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• Upwards and downwards triangles after the active field name
• Making field names of the active field bold/underlined
• Change the color of the active field

What's considered the best method to indicate an active table field?

In my opinion using upward and downward triangles is the best way, because they are unambiguous and easy to interpret.