Melhor e ideal maneira de juntar o valor máximo de outra tabela

Não faça dessa forma, aqui está uma maneira melhor, primeiro, adicione um campo para armazenar na tabela de usuários o último ID de qualquer nova entrada de log, depois crie um gatilho que atualize esse campo em cada novo campo inserido, depois ( opcional) crie uma visão para ter uma “tabela” completa para consulta.

-- Add new field to save the modified id
ALTER TABLE `users` ADD `last_mod_id` INT(11) NOT NULL, ADD INDEX (`last_mod_id`);

-- Drop the trigger just in case it exists
DROP TRIGGER IF EXISTS `update_last_id`;

-- Create a trigger to save the last modified id each time a row is created
DELIMITER ;;
CREATE TRIGGER update_last_id 
    AFTER INSERT ON user_log
    FOR EACH ROW
    BEGIN
        UPDATE `users` 
        SET `last_mod_id` = NEW.id
        WHERE `id` = NEW.idrel;
    END;
;;

-- Emtpy your table
TRUNCATE `user_log`;

-- Create all new registries 
INSERT INTO `user_log` VALUES (1,1,'log1','2014-01-01 13:00:00');
INSERT INTO `user_log` VALUES (2,1,'log2','2014-01-02 13:00:00');
INSERT INTO `user_log` VALUES (3,2,'log3','2014-01-03 13:00:00');
INSERT INTO `user_log` VALUES (4,2,'log4','2014-01-04 13:00:00');
INSERT INTO `user_log` VALUES (5,3,'log5','2014-01-05 13:00:00');
INSERT INTO `user_log` VALUES (6,3,'log6','2014-01-06 13:00:00');


-- Query
SELECT
    u.id,
    u.name,
    u.last_mod_id,
    l.event,
    l.log
FROM
    `users` as u
INNER JOIN
    `user_log` as l
    ON (l.id = u.last_mod_id)

-- Drop view if exists
DROP VIEW IF EXISTS `users_log_view`;

-- View
CREATE VIEW `users_log_view` AS 
    SELECT
        u.id,
        u.name,
        u.last_mod_id,
        l.event,
        l.log
    FROM
        `users` as u
    INNER JOIN
        `user_log` as l
        ON (l.id = u.last_mod_id);

-- Query
SELECT * FROM `users_log_view` WHERE `id` = 1;