It depends on the media, the burner and the burner's firmware.
In general I would recommend that for best results you burn media at the speed it's rated at, or down to half that speed - but not lower than half.
Example: If you have 4x media you should burn at 4x or 2x.
Example: If you have 8x media you should burn at 8x, 6x or 4x.
If you REALLY want to know what's best, you'll have to burn media at all possible speeds in your burner and then test the burns by scanning with a PIE/PIF capable drive. If you don't want to do this or don't know what the he** I'm talking about, then just go with my general recommendation.