I've had a hard time paying attention and appreciating commentaries on movies, but the first one I actually had fun with and listened to more than once was the Repo: The Genetic Opera commentary will Alexa Vega, Bill Mosley, Ogre, and Darren Lynn Bousman.
I have found the commentaries on John Carpenter's films *esp. the ones with Kurt Russell* to be both fun and informative i.e. "Big Trouble in Litte China", "Escape from New York", & "The Thing". Also practically a film school class Roger Ebert's commentary on "Dark City" is very informative. As are commentaries by Quentin Tarantino & Robert Rodriguez.
Rodriguez' esp. ...they are like Film School *the kind where you actually learn some "practical" things*.
Sounds cool! The Repo commentary was great because they kept telling you about inside jokes, Darren Lynn Bousman pointed out everything he recycled from SAW to use in the movie, and you get to hear Alexa Vega talking about things like how Darren forced her to drink the "medicine" (which was a mint in water) and that it gave her minty farts. They even go over times that the set caught fire. The four of them have a really great time and you learn so much about the movie.
Ebert also does a good commentary track for Citizen Kane. Even though he's mostly regurgitating EVERYTHING I've heard in all the film classes I've ever taken, his commentary makes for a great guide for those who are new to it.
I enjoy most of the Pixar commentaries (especially the animators commentaries on Finding Nemo, The Incredibles, and Wall-E).
The one commentary that actually makes a movie better was the writer's commentary for Star Trek Generations. They basically rip into the film and explain how the whole ordeal was a huge mess of producers and continuity.
My favorite commentaries are the Lord of the Rings Extended Editions for serious commentary by the cast and crew, and there are 4 different commentaries for each movie, and UHF which just makes my day. I mean at certain points, "Weird Al" Yankovic will actually jump up in front of the screen, not to mention it's mainly just him and his manager/director Jay Levey with guest appearances by Emo Phillips, Michael Richards, and a phone call to Victoria Jackson.
Paul Thomas Anderson's directors commentary for Boogie Nights is one of my favorites that I just watched for the second time last night
It's a shame there's no commentary track on Magnolia, Punch Drunk Love, or the overall shit release for There Will Be Blood
Matt, Jack, & Jon are three eloquent young gents. They philosophize, externalize, and harmonize on the most pressing subjects of the modern age. Actually, they mostly just talk about movies and current events, but the presentation is what counts.