Numbered variables are a bad idea. Using numbered variables will make writing code harder. So the first thing to do is put all of those C1s into one variable M:
>> M = [c1;c2;c3]
M =
1 5 6
3 2 0
4 7 0
Then your task is trivial using bsxfun, sum, and max:
>> [~,idx] = max(sum(bsxfun(@eq,M,b),2));
>> [M(idx,:);b]
ans =
1 5 6
3 5 6
