How to get a unique highscore per user
Pull the time from the highscores table separately
MySQL doesn't respect ANSI standard aggregations by default: hence your dodgy data

SELECT 
    u.UID, u.ALIAS, u.EMAIL, h.SCORE, h.value1 AS TIME
FROM 
   (
    SELECT user_uid, MAX(score) as SCORE
    FROM highscores
    GROUP BY user_uid
   ) x
   JOIN
   highscores h ON x.user_uid = h.user_uid AND x.SCORE = h.SCORE
   JOIN
   users u ON h.user_uid = u.uid
ORDER BY h.SCORE DESC
LIMIT 0, 10


This assumes that each user does not have multiple high scores with the same value.
If you do, you need a 2nd derived table with an aggregate to nest the x/h JOIN