Carp are not as picky about the fly as they are about the presentation. In many ways, they are more like trout than most would like to admit. If the fly is the same size as food and swims like food, they will eat when they are in the mood.
Having said that here are a few good ones:
-wooly buggers in black or brown
-backstabbers
-Headstands
-Clouser Swimming Nymph's
-Blood Dot's
Tying instructions for all can be found on youtube