I am playing a Warlock, I highly recommend multiclassing after Level 9 or 12 if you really want that feat.
Warlock is a really great class to multiclass no matter the build and since you don't get anything even as good as any other class after level 9 EXCEPT Eldritch invocations. Your spell slots don't get any higher so you don't get your choice of higher level spells or the ability to cast your lower spell at a slot level above 5. Mystic Arcanum are better then nothing but in no way make up for the lose of higher level spell slots. I could recommend any caster class providing lower level spell slots which makes using more efficient use of hex and if you get to level 4 or 5 slots way more casting options than the mystic Arcanum though you do lose all hope of casting any spells above 6.
On the other hand if your not going to a primary caster and magic/eldritch invocations are used as melee support, skill junky options, or even crowd control abilities then I would recommend Paladin or Fighter(Eldritch Knight) for melee both providing spell slots for more flexible spell casting and lvl 5 spell slots that recharge on short rest are great for smity paladins with cure wounds while Eldritch Knight's melee prowess and multi attacks combine with warlock pact of the blade and/or enhanced eldritch blast just makes a stronger Eldritch knight.
For Skill junky I mean ... Bard...charisma... spell slots and skills for days also because of cure wounds with 2 level 5 spell slots that recharge on a short rest you can heal the group with those remaining slots before any short rest to keep group health up.
For Crowd control, I recommend Sorcerers, again charisma based and some good meta magics which help of set the lose of a few eldritch invocations.
I get some of the eldritch invocations you get at lvl15+ are really good but if you compare them vs what you get from 8-11 levels of another good class features its really hard to stay warlock all the way, in my opinion. IF your going Pact of the blade, Hexblade or Fiend, you can hold out to Warlock level 15 and get all but one Eldritch invocation (access to level 15 evocations) while still multiclassing for spell slots and/or multi-attack at level 5, which can replace the warlock invocation thirsty blade meaning you don't really lose ANYTHING by going Eldritch Knight in trade for one eldritch invocation. If you go Eldrich Knight to level 6, you lose level 15 eldritch invocation access but you get your feat back... If you go 9 warlock, 11 Eldritch Knight you get 3 attacks per action, the standard 5 ASI/Feats, and 5 of 8 Eldritch invocations.... Not to shabby.
Again... In my opinion... multicasting warlocks some where between lvl 9 - lvl 17 is almost just common since, this class dies after 17. If you go paladin "smite knight" you can leave warlock at level 7 or 8, but if I started with warlock I would avoid paladin and get the eldritch smite invocation instead. If I started paladin and was just looking to maintain with the group 8 level or warlock would be nice.