Sun and Jin's death certainly rivaled Charlie's and Juliet's death as the most emotional scene in Lost yet. But after thinking about it, in my opinion, Sun and Jin's death is the saddest moment on Lost yet. Jin was free to go and save himself, but he decided that now that they were finally reunited, he would never leave Sun again. So powerful. Jin died a courageous death. It really pains me to think that Jin will never meet his daughter who he only just saw in pictures a couple days ago.
The part that makes these three death scenes (Sun&Jin, Charlie, and Juliet) are the emphasis the camera places on their hands as they die. Sun and Jin were holding hands until death, and only let go after both of them succumbed. One of the most powerful shots of last year's finale for me was the shot of Sawyer and Juliet holding hands as she slipped down into the Swan drill shaft, then letting go as she plummeted downwards. And nobody will ever be able to forget the iconic image of Charlie's hand going up on the door of the control in the Looking Glass telling Desmond "Not Penny's Boat," then Desmond putting his hand up next to Charlie's. The hands for some reason are what always get me about these emotional deaths.
If you can make an argument for a scene other than Sun and Jin's death being the saddest yet, I'd like to hear it.