I've really stopped watching commentaries the past few years, but I used to throw them on every now and again...
BEST
I'll echo the sentiments for Fincher's solo commentary on FIGHT CLUB. Very solid.
Jay Roach and Mike Myers were pretty good on the first AUSTIN POWERS film.
Ben Stiller and Owen Wilson (I think he's on it, too) for ZOOLANDER (which also wins the title for funniest menu audio).
Verhoven's Robocop commentary is good, too. A nice introduction into his warped European mind.
And albeit a bit droll, I enjoyed Nicholas Meyer's WRATH OF KHAN commentary and McTiernan's for the first DIE HARD.
WORST
Any Mel Brooks commentary. Years ago I tried listening to the MEN IN TIGHTS commentary on laserdisc. Awful. Dom Deluise appears on screen. "Here's my good friend Dom Deluise." Yeah, no shit, Mel. This ain't radio, we see him, too.
The Star Wars ones are boring, and full of revisionist Lucas history. But they're still not as bad as the definitive collection laserdisc commentary, which was about two minutes of talking for every twenty minutes of screentime.
The absolute worst was W.D. Richter's BUCKAROO BANZAI commentary. He even had one of the secondary actors (Rico? I can't remember) there in character(!?). And yet no one explained to them that commentaries involve speaking. They talk over opening credits then...silence. I skip to next chapter, maybe in the next scene there's something worth commenting on...nope, silence. Next chapter? Silence. Fed up, I turned it off.