I'm going to go with the nameless ensign from Voyager's "Displaced". From her hilariously over-the-top stunning to the worst line readings ever she fails on every level imaginable.
Jump to 4:20. Her scenes last for 20 seconds.
No wonder she was gunned down: if you're trying that hard to fire a phaser, you're doing it wrong!!! What kills me though is that apparently, "Ensign Lang" here isn't just a yellowshirt, but a semi-recurring character. Ugh.
On the other hand, I can't decide if 1:20-1:40 is one of the worst or one of the greatest feats of acting in all of Trek: http://www.youtube.com/watch?v=mx-jB...eature=related