Whats the last level in?

[krugaan]

You need 17 wiz for wish/simulacrum and 2 lock for eldritch blast / agonizing blast...

That leaves one level. What is the best single level you can take for maximum benefit?

For extra humor, we're going to insist this last level not be wizard for that extra high level spell slot.

[JeffreyGator]

For only 1 level I think clerics, fighters, bards and monks have the most options.

Probably animal, arcane or knowledge cleric.
all gives medium and shield and extra proficiencies and give some more spell/cantrip options.

since I really want at least 2 level dips on the fighter, monk and bard I like cleric the best for this.

eventually your simulacra will run out of prepared spells and might thank you for having known more cantrips in addition to their spellbook of every wizard ritual.

Another good option for that is one more level of warlock to get 2 extra cantrips and ritual everything book.

I actually think that the 18th level wizard 1st and 2nd at will spells are much more useful to the simulacra than the extra slot or almost anything else.

[RulesJD]

You need 17 wiz for wish/simulacrum and 2 lock for eldritch blast / agonizing blast...

That leaves one level. What is the best single level you can take for maximum benefit?

For extra humor, we're going to insist this last level not be wizard for that extra high level spell slot.

1. Wild Magic Sorcerer. The ability to generate advantage nearly at will and Wild Magic surges permit a high level Wizard to do things they wouldn't be able to do otherwise. Plus your Simulacrums will be able to trigger Surges as well for double the effectiveness.

2. Alternatively, Cleric (Life or Light). Medium Armor + shield proficiency + Guidance + Bless + Healing Word is still useful at all levels. The use of Guidance + Healing word easily outdoes what Fighter levels would give you. Life for the added Healing from upcast spells (not attribute dependent) or Light for access to Faerie Fire (ridiculously useful 1st level spell when combined with Animate Objects from your Simulacrum (or vice versa rather), Animate Dead, etc.

[Vogonjeltz]

You need 17 wiz for wish/simulacrum and 2 lock for eldritch blast / agonizing blast...

That leaves one level. What is the best single level you can take for maximum benefit?

For extra humor, we're going to insist this last level not be wizard for that extra high level spell slot.

Are we trying to make this the least effectual character of all time?

In that case I'd say a level of something that adds at least one more mandatory stat (to spread ourselves extra thin on the stat array).

Monk maybe because then you have to have a 13 in Dex, Wis, Cha, and Int. It does net you unarmored defense and any attempt to undergo melee combat would be ludicrous on the build already so Martial Arts may as well not exist. Plus with only one level you don't even get ki powers.

Alternatively, Barbarian with Rage has negative synergy and it requires Str which has next to no benefit at all for the Wizard or Warlock.

[Gtdead]

Wizard 18 for spell mastery. If you manage to go 17, it will be a shame to not get it.
On an other note, why not do this as a sorcerer? He still get access to wish->simulacrum, throwing 4 eldrich blasts per turn.