Flame Strike -- Dex save for damage, for slower opponents (or, Hideous Laughter for a Wis save, but with less damage)
Spare the Dying -- for when the Cleric gets hit!
Shilleleigh -- for a Charisma-powered d8 quarterstaff.
For my money Guidance, Vicious mockery or Frostbite and Minor Illusion are the best three... since you might already have Minor Illusion though, hmm how about Mold Earth which has a lot of great utility. Although personally I went with Thaumaturgy for roleplay reasons.
Spare the dying can be replaced by having healers kits. The healers kit price is insignificant when compared to a wasted cantrip. Guidance is a good one if the cleric doesnt have or forgets constantly to use it. Go for utility, as a warlock you already have the best damage cantrip in the game.
Unless if for some weird reason you are going melee, then shillelagh and/or blade cantrips from SCAG are options.
Thorn whip from the druid list. If you hit you a get a 10 ft. forced movement pull with no save. Combine with the repelling blast invocation (a 10 ft. forced push w/no save) for a little taste of 4e-style combat in 5e.
The best use is to pickup spells that you otherwise can't access. Except for Guidance (you can never have too much Guidance), you should pretty much only consider spells the rest of your party can't cast and that aren't on the Warlock list. I'll break them down into utility and combat, since you'll probably want one of each (plus Guidance... have I stressed how awesome that is enough?).
Vicious Mockery - very little resistance/immunity to psychic damage, disadvantage is super strong until opponents get multi-attack
Sacred Flame - might be useful against low dex/high ac opponents, but in general Eldrich Blast is going to be your go-to damage dealer
Thorn Whip - pretty solid control, and since it may not come up often, shouldn't fight with EB as your primary attack
Shocking Grasp - kinda weak for your needs, but sometimes a get-out-of-melee free spell
Dancing Lights/Light - unless the party all has Darkvision, I've found that one of the spells per group is a good idea
Mending - fixing broken/damaged things, assuming you have a flexible DM, can provide a lot of extra information when delving old tombs and such
Message - I never see this come up much, but it fells SOOO good when it does
Resistance - not a fan of this spell, because you almost never get a chance to cast it before you need it