either ek 3, 7, 11 or 14. Maybe 16 for another stat increase and you don't lose too much on the spellcasting side because you have 3rd level spells from the eldritch knight. For some schools the level 6 benefits may not be worth giving up a stat increase and what else you get with fighter 15. But if you go that high, you may as well take level 17 and 18 too. You still have the big level 2 feature of the mage and access to level 4 spells and a spellbook with quite a few useful spells actually. Elven bladesinger may come to mind as does evoker which makes spamming fireballs trivially easy.