Easily Fred and Sirius, on equal measure.
Mainly because of how disrupting they were; Harry's only thing close to a family in the world dying, after only being in his life for such a short amount of time was tragic I thought, and Fred because... well it wouldn't be as sad if both twins died, but it's just so hard to imagine them as not together, as just 'George'.
How the deaths were portrayed though, the last film fucking destroyed all the drama of the Battle of Hogwarts in favour of spectacle, and they didn't even include Fred dying as he's supposed to, they just see his body in the great hall.
So Fred's book death was far better.
However Sirius' death in the film was easily better, in fact the whole 5th film was.
In the book it prolongs it and tries to explain that he fell through the spirit wall or whatever, while in the film it's simple and crystal clear that he was flat out killed.
As a side note, I thought the scene around Snape's death was hilarious in the film because of how big an error they made:
Snape wants to see Harry's eyes; his bright green eyes, that he is famous for taking from his mother.
Cue flashback to Snape and Lily as children, with Lily's dark fucking browneyes.
I mean, come on.