Well, let me jump in.
My favorite would be The Big Lebowski, although I fully confess that part of it's charm is that I think I hung out with those people for a few years...
I would also rate Oh Brother and Fargo extremely high.
Millers Crossing I'd put pretty high, but not top tier, along with Blood Simple.
Intolerable Cruelty I would also rank as an "adequate Hollywood comedy", as Kalo puts it, but I expected more from the Coens. Perhaps I was just overrated them, though. I'd put Raising Arizona about here as well. Okay comedy, but nothing worth raving about.
The Man Who Wasn't There and the current Burn I'd put it the "don't bother to see" pile, something that shouldn't exist if the Coen Brothers were as good as their reputation.
I must confess to not seeing Hudsucker, No Country or Barton Fink. Yet. The Ladykillers I have no interest in seeing, having thrown it into the trash bin I have labeled "Pointless Remakes". I mean, damn; the original was a classic...why bother?