SELECT *
FROM forum_category AS c
  LEFT JOIN (
SELECT category_id, title,
      (
        CASE WHEN t.timestamp > MostRecentThreadSubquery.LatestReply THEN t.user_id ELSE MostRecentThreadSubquery.user_id END
      ) as MostRecentUserID,
      (
        CASE WHEN t.timestamp > MostRecentThreadSubquery.LatestReply THEN t.timestamp ELSE MostRecentThreadSubquery.LatestReply END
      ) as MostRecentTimestamp, num_replies
    FROM forum_threads AS t
      LEFT JOIN (
        SELECT thread_id, user_id, timestamp as LatestReply, (SELECT COUNT(*) FROM forum_replies WHERE thread_id=r1.thread_id) AS num_replies
        FROM forum_replies r1
        WHERE timestamp = (
            SELECT MAX(timestamp)
            FROM forum_replies r2
            WHERE r1.thread_id = r2.thread_id
          )
        GROUP BY thread_id
      ) AS MostRecentThreadSubquery ON t.id = MostRecentThreadSubquery.thread_id
  ) AS MostRecentPerThread ON c.id = MostRecentPerThread.category_id