A rare posting for me over on the Marvel thread about the Punisher, got me wondering how many of DC's super heroes would fall to a top quality non powered assassin...say somebody who would be employed by likes of Lex Luthor to take out a troublesome foe. Professional, big fee, access to good quality weaponry, but no access to really, really outlandish technology. (Lex willing to pay a big fee, but not willing to supply weapons in case attack traced back to him.)
I take it as an article of faith that such an assassin could take out any member of the Bat family...up to and including Batman himself. If that can't be done...then let's stop pretence that such characters really are "normal" humans. Obviously same would go for likes of Green Arrow.
At other extreme...I ddon't think a non powered assassin, without outlandish technology, could take out one of the power houses.
Yes, right at top of the tree Superman has weaknesses...but he also has extra-ordinary senses to forewarn him...and most of the weaknesses such as kryptonite are usually shown as not readily available. Any GL...with way ring is usually written (as sensing danger automatically and throwing up force field)...ought to be proof against such an assassin. Flash? Not sure...if you took him by surprise...when out of speed trance...perhaps he'd fall. Could go on...but personal gut feeling is that the true power houses can't be taken down in this hypothetical scenario.
But the mid tier guys? The likes of Hawkman and Atom? I think a really skilled assassin would take them.