I really don't understand how people think the first 'no' actually "isn't that bad." In my opinion, it's possibly worse than the second one for a couple of reasons.
First, the line delivery is absolutely awful, the pitch raise at the end indicates some sort of questioning . . . "no?" . . . which to my ears just sounds ridiculously stupid and out of place for Vader's character (particularly ignoring the prequel trilogy and going by the portrayal people would have seen up to that point in the films release order).
Second, the first "no" sounds horrible in all the mixes I've heard, too loud and sounds obviously pasted into the scene.
Third, it covers up the sound of Luke being tortured and, in particular, covers up him calling out "father" - which, imo, is a far more effective cue for something that would finally get through to Anakin in that moment. In fact, it's almost like Luke says, "Father!!" and Vader goes "No? I don't think I'm your father." It's ridiculous.
Along those same lines, the sounds of Luke's torture lets you visualize the scene Vader is seeing as his son is tortured, and that shot of just him looking lets the audience fill in his conflict with much more effectiveness than either of these added lines manages to do. The dub not only makes Vader sound like he's making some funny questioning face under that mask, but also ruins the impact of hearing Luke that always stood out to me during that shot.
The entire sequence on the Deathstar between Luke, Vader, and Palpatine is some of the most well scored and epic stuff in the OT, and to cover up that great music with either "nooooo" is unacceptable. Imagine if they'd dubbed some horrible father/son banter between Luke and Vader during their climactic saber fight under the stairs after Vader threatens Leia, Luke attacks, and and the chorus kicks in with possibly the most amazing musical cue in the entire trilogy.
http://www.youtube.com/watch?v=ymYj4P_yRyo#t=1m09s
