This Top Ten Bitter list is from the 2012 U.S. Open Beer Championship and from the reviews from Beer Advocate. (Bitter is not a separate category at the GABF or the World Beer Cup).  
 
1. Butler's Bitter - Niagara College
2. Arkell Best Bitter - Wellington County Brewing
3. Neustadt Bitter - Neustadt Brewing
4. Uncle Teddy's Bitter - Victory Brewing Company
5. Honker's Ale - Goose Island - Illinois
6. Younger's Special Bitter - Rogue Ales
7. Bitter Brewer - Surly Brewing Company
8. Best Bitter - Dick's Brewing - Washington
9. Royal Oak Traditional Bitter - O'Hanlon's Brewing - England
10. Bluebird Bitter - Coniston Brewing Co

 

          

Bitter
Subcategory: Ordinary Bitter
Ordinary bitter is gold to copper colored with medium bitterness, light to medium body, and low to medium residual malt sweetness. For the purposes of this competition, either English or American hop flavor and aroma character may be evident at the brewer’s discretion. Mild carbonation traditionally characterizes draft-cask versions, but in bottled versions, a slight increase in carbon dioxide content is acceptable. Fruity-ester character and very low diacetyl (butterscotch) character are acceptable in aroma and flavor, but should be minimized in this form of bitter. Chill haze is allowable at cold temperatures.
OG: 1.033-1.038
FG: 1.006-1.012
Alcohol by Weight: 2.4-3.0%
IBU: 20-35
Color SRM: 8-12

  

Subcategory: Special Bitter or Best Bitter
Special bitter is more robust than ordinary bitter. It has medium body and medium residual malt sweetness. It is gold to copper colored. Hop bitterness should be medium and absent of harshness. For the purposes of this competition, either English or American hop flavor and aroma character may be evident at the brewer’s discretion. Mild carbonation traditionally characterizes draft-cask versions; for the purposes of the bottled entries entered in this competition, normal or a slight increase in carbon dioxide content is acceptable. Fruity-ester character is acceptable in aroma and flavor. Diacetyl (butterscotch character) is acceptable and characteristic when at very low levels; the absence of diacetyl is also acceptable. Chill haze is allowable at cold temperatures.
OG: 1.038-1.045
FG: 1.006-1.012
Alcohol by Weight: 3.3-3.8%
IBU: 28-46
Color SRM: 8-14