(Not counting Crimewave)

Blood Simple
Raising Arizona
Miller's Crossing
Barton Fink
The Hudsucker Proxy
Fargo
The Big Lebowski
O Brother Where Art Thou
The Man Who Wasn't There
Intolerable Cruelty
The Ladykillers
No Country For Old Men
Burn After Reading
A Serious Man
True Grit

.....that's a pretty impressive spread of movies.

Personally I'd go for

1. Fargo / Lebowski / Barton Fink / No Country For Old Men
5. Hudsucker Proxy
6. A Serious Man
7. True Grit
8. The Man Who Wasn't There
9. The Ladykillers
10. Burn After Reading
11. Miller's Crossing
12. Intolerable Cruelty
13. Raising Arizona
14. O Brother Where Art Thou
15. Blood Simple

I genuinely can't separate the top four... depends which day it is. They're all amazing for different reasons.