I like so many breeds, I'd have a terrible time choosing just one! In fact, I have a mixed batch arriving in June!
Aside from that, I love looking at the mixed flock wander about; they are as soothing as a fish tank, IMHO.
Also, as you said, different breeds do have different qualities.
Just as different tomatoes/tomato plants have slightly different qualities to recommend them, which is why I'm growing so many varieties again this year!
Mostly I do stay with my heavy breeds, though, like the Buff Orpingtons, Black Australorps, etc., though I do have some EE's. I have found that the mellower breeds tend to be happier with other mellow breeds in their midst. Still, I managed to find 10 breeds to fit the bill!