.So far the most overpowered spell that I've noticed is Eldritch Blast. Did warlocks really need an attack cantrip that much better than the other cantrips?
Eldritch Blast isn't much better than Fire Bolt on its own, and any caster can pick it up via Spell Sniper or Magic Initiate (though it's most useful for Sorcerer or Bard since it will use Cha).
Really it's the Invocations that make it strong, and those are directly balanced against the warlock getting fewer spell slots. For example, compare a warlock and a wizard at 8th level.
Say that a typical adventuring day has six combats of three rounds each, with two short rests. The wizard has 12 spell slots, plus up to four more from Arcane Recovery. The Warlock has 6 over the course of the day, with more constraints on when he can use them.
So when a warlock is out of spells and falling back to Eldritch Blast, the wizard is falling back to low-level spells. I think the fair comparison is Eldritch Blast vs. a spell a couple levels below your max (e.g. Scorching Ray at 8th level).