Then I'd advise sticking with EK until you get to 14th level so that you can get haste.
That's fine from a combat effectiveness standpoint, but the OP seems to want to multiclass for some reason, and there's no guarantee the campaign will go all the way to 20. Best do it early enough to reap the benefits. If the OP stays fighter till 14, then MCs into diviner in order to get Portent, and the campaign ends at 15, s/he will be a Sad Panda. (And it's not like picking up wizard levels now will prevent you from taking more EK levels later.)
I agree with the general sentiment that it's insane to multiclass one level shy of your third attack. Stick with fighter till level 11. After that, wizard is the obvious class to MC into, and the big choice is which tradition you go with. The level 2 tradition feature is the important thing here. You are unlikely to get to the level 6 feature, or to enjoy it very long if you do.
If you're allowed to use Xanathar's Guide to Everything, the choice is clear: War Magic. +3 to initiative
and you can give yourself +2 to AC or +4 to any saving throw, as a reaction, at will, no usage limit. (It does prevent you from casting non-cantrip spells on your next turn. OH NOES. You'll just have to walk up to the enemy and Cuisinart them with your three attacks per round, how sad.)
If you don't have or aren't allowed XGtE, your best options are Abjuration for a batch of quasi-hit points that refreshes every time you cast
shield; Divination for the ever-popular Portent; or Enchantment or Illusion if you'd like to bolster your capabilities outside of combat.
For spell selection, focus on utility magic. Do
not take damage spells; in a level 12+ campaign, low-level damage spells are a waste of your action*. But
feather fall,
charm person,
find familiar,
Tenser's floating disk,
alter self,
detect thoughts,
invisibility,
see invisibility,
misty step,
suggestion... those will serve you well even at high levels.
[size=-2]*Unless the campaign runs long enough for you to reach wizard 5.
Fireball is so good that it's worth having even at level 16.[/size]