SELECT cat, count(cat.id) AS am
FROM 
  Category cat, IN (cat.contributions) c
WHERE 
  c.user=:user
GROUP BY 
  cat.id
ORDER BY am

----------------------------------

SELECT cat
FROM 
  Category cat, IN (cat.contributions) c
WHERE 
  c.user=:user
GROUP BY 
  cat.id
HAVING count(cat.id) = max(count(cat.id))