Is it possible to only display the largest calculated table column?
Atleast for 8.0 there's the GREATEST function. I would also suggest that you do one scan against the table instead of six:

SELECT GREATEST(SUM(CASE WHEN vote_candidate_1 = 1
                         THEN vote_candidate_1 END)
              , SUM(CASE WHEN vote_candidate_2 = 1 
                         THEN vote_candidate_2 END)
              , SUM(...)
               ) as ...
FROM votes;