So we have these items today. Spoiler tags on recommendations for each:

DAY 1 SPOILS

STR+ - This should go to Lightning - My ability reduces my stats in strength. As such I will need more strength to effectively use this
MAG+ - This should go to Ashe - Ashe's ability reduces her stats in magic and makes her powerful weak. A boost to this will reduce incoming attacks
SPD+ This should go to Yuna With the lowest overall speed stat (except Lightning) this will boost her up

Hi-Potion This should go to Rinoa - Unpredictable fights could use for heals

Chocobo Feather - consumable and adds 40 to your SPD stat This should got to Zack - The haste ability will allow his speed to go to 120 and then attack.