Let the right one in - The commentary between the author and the director is warm, engaging and quite funny in places, a genuine sense of friendship between the two, with a dry sense of humour.
Have to echo Carpenter/Russell commentaries, my favourite being Big Trouble in LIttle China, as you may have guessed.
Dog Sodiers cast commentary is aces too, lots of insight and banter.
Commentaries bring me back to mediocre films that have congealed in my collection, a good commentary is a great addition to a dvd and is always the first extra I look for.
Apart from Will Ferell ones where the staged comedy is just too forced and comes across more annoying than funny.