What are you trying to optimize?
Want some big blasty spells? Go fiend.
Want to go melee? Grab another class first for armor and weapons, go fiend or undying light for the spells or great old one for Entropic Ward or Archfey for Greater Invisibility and Fey Presence. (Plus look at tome lock for shillelagh with some SCAG cantrips vs bladelock - CHR for weapon attacks + BB/GFB is arguably better in melee than a bladelock.)
Want battlefield control? Great old one probably is your bet. Want single target DPR? That can be done by all, but fiend gives you temp HPs for dropping opponents.
I think you get the point.
So "best for optimization" ends up with "they each have their strengths, you'll probably be the most optimized when you are synergized well with your party". Figure out what would be a force multiplier for them and you can pick a warlock pact to help.
All of that said, I'm right now putting together an undying light warlock in case my current character dies in one game. They have less spells added to the list, but Radiant Soul buffs a bunch of them, the other powers fit the group, and thematically he'll get along well with the paladin of Lathander.