A tie for me as the most emotional death ever in Trek. The one it's tied with is in your future. Gut wrenching. Especially what the Xindi says to him as he stabs his last breath from him.
A woman I worked with during the ENT run was a big fan. On the morning after this ep, I went directly to her cubicle when I arrived at the office. She looked up as I peered in and said only one sorrowful word, "Degra".
He had become a favorite character and we were both shocked by his departure and the method thereof.