Quote:
I kinda see what you mean. It seems odd to me too. Maybe I just accept that those things are just normal for the breed its-self. Unless one really tracks the lineage of their flock, it would be very difficult (if not impossible) to tell them apart from simple mutts of various breeds. I just figure if it is normal for the breed to have various combs and colors, then thats just the way it is. I know it is a difficult concept to embrace when so much emphasis is always placed on having a certain standard to adhere to when it comes to breeding and showing, not to mention just for being able to visually determine a breed w/out genetic testing or lots of paperwork trailing particular breedingstock birds.
Is it impossible to have a true breed when the only real defining physical characteristic is egg color? In that way, Icelandics all share a common trait. 
It reminds me of the EE questions seen alot here- Like "Are EE's a breed,or just a mutt"? "How to determine if your EE is pure, or just a barnyard mix"? etc.
I have to agree with what Suzette is saying here.  Icelandics are sort of like the Norwegian Jaerhon in that they are a breed that have been around for a while in another country and recognized as "a breed".  However, we get it into our heads after looking for the APA Standard in a breed that we fail to recognize that some of the breeds that have developed, for whatever reason the continue to be bred, have variations within the breed that are not selected or de-selected for.  You should also consider, in many countries where some of these birds are coming from, people have chickens more for practical purposes than for hobby.  Especially when you are talking about a small island country like Iceland.  Why would they bring in several breeds of chickens when they have some right there?  They have our version of Backyard Special, but they are all Icelandics because that is where they come from and probably have some common genetics.  Does that disqualify or qualify them as a breed?  Who decides?
Does the OP want to know of ALL chickens, who is the rarest in the US, or just APA recognized breeds, or what?
It's funny you should mention the Jaerhon, as it's another victim of the modern obsession with color standardization. When the few remaining Jærhøns were rescued from oblivion, around 1900, they came in many color variations, like the Icelandic and many of the old Swedish breeds. The government preservation program felt that it was important to become "modern" and settle on a single stable color pattern, which was eventually achieved... at the expense of genetic diversity, :-( 
For more, see 
this site in Norwegian, or 
Google Translated ... and some other, shorter articles from the 
Norwegian Genetic Resource Center and the 
Landsbrukmuseum. Fascinating,
"Rooster  No. 1" and "Hen No. 26"...  :-(  
Best - exop