Well, it definitely wasn't Ben Reilly. That was the most unsatisfying/least visceral comic book death I've ever read. I mean really, he get's impaled and turns into a pile of goop... come on!!
I'll go with Superman as the most memorable (yeah, I know it was a marketing ploy, but it was done well)
Most important - Flash (Barry Allen)
Most significant - a tie between Bruce Wayne's parents and Uncle Ben
Maybe Donna Troy would fit in somewhere. Even though like most Comics characters, she didn't stay dead.
Bucky's apparent death was big for a lot of fans until his recent return.
http://www.youtube.com/watch?v=ETvF91Cme-0
Death of Angel Character from RENT.