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?

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