Aqui está:
SELECT i.title, bmax.amount, u.username
FROM itembid AS b
JOIN (SELECT item_id, MAX(amount) AS amount
FROM itembid
GROUP BY item_id) AS bmax
ON b.item_id = bmax.item_id AND b.amount = bmax.amount
JOIN item AS i ON i.id = b.item_id
JOIN itembid AS ubid ON ubid.item_id = i.item_id
JOIN user AS u ON u.id = b.user_id
WHERE ubid.user_id = :current_user
