Drawmij's Instant Summons is pretty bad. It's not useless (you can cast it on your spellbook) but it costs a lot, and it doesn't do very much compared to what you would expect a 1000 gp 6th level spell to do. In many of the cases where you'd actually want the spell to work (someone took your spellbook and you want it back), it in fact does nothing except let you know who's got your stuff and sort of where.
My rationale for bad spells like this is to assume that some wizard needed the spell and just slapped it together by throwing resources (gp and spell slots) at it to reduce the research time. If you're only going to cast the spell once, and you already happen to have 6th level spell slots and a bunch of sapphires, you have no motivation to spend extra time optimizing the spell once you've got something that basically works.
As others have said, Witch Bolt, Mordenkainen's Sword, and True Strike are also pretty bad. True Strike is notable for the fact that almost every time you think you've found an actual use for the spell, a closer read of the spell reveals that there is some kind of restriction preventing it from being useful in the scenario you're imagining. In fact, the ONLY scenario I can think of where it's useful is when an mid-level Eldritch Knight is trying to cancel disadvantage against something with high AC and no other easy way of cancelling disadvantage, like an ancient red dragon with both Darkness or Blur up and the ability to cast Shield. In that scenario, one attack without disadvantage could actually be more valuable than three attacks at disadvantage.
Of course the EK in that scenario is going to die either way--you can't kill an ancient red spellcasting dragon with one puny attack per round--except in the even more niche scenario where he's got some kind of super nasty poison on his weapon and the dragon has used up its legendary resistances.
So yeah, True Strike is probably the worst spell in the PHB.