Whoa back track... Invincibility means immortality? Don't think so. Invincible means not being able to be harmed ,nor feel pain, but can still die. Immortal means no age, poison, or physical damage can kill you but you can still be torn apart into several pieces and still survive but are in no way impervious to physical damage. Now the real one you would want in invulnerable because you are immortal and invincible at the same time. Sorry I just had to clear up now.
The worst power ever huh? Well I would think that the worst would be super speed that you couldn't see what was going on around you as you moved. Actually super speed in general isn't that good because what qualifies as super speed? Are we talking about superhuman or supernatural? Superhuman would be like running at the highest point possible and just over what is normally humanly possible. Supernatural would be going well over what is humanly possible and then some. If you were going supernatural speed then your body would probably be tron apart from hitting things as you run. Superhuman would probably only get you to about whatever Captain America can run and then just a tiny but more.
The worst would be that you could die, but you come back to life. Think about it. You would feel all the pain, then have to go through it again in a little while.
Have any of yous seen Torchwood: Miracle Day? It's basically whereby the entire planet Earth is immortal apart from Captain Jack Harkness and everybody can feel pain but cannot die. Ironically it was Captain Jack Harkness had the superpower: The ability to die. See, people who can't die are the majority therefore if you do something that nobody else can, you've got a superpower and a **** fine one too... See the immortality is a rubbish superpower and it was worse because it was worldwide therefore people were suffering. People were getting brain damaged and still not dying. I suppose that makes it the worst superpower. Immortality without the removal of pain and without instant pain recovery...
Having your face grow thousands of huge pimples and they explode all over ur enemy... And yourself. That would be one of the first heros who would never get the girl