I was surprised by how much I liked Adam Baldwin's voice acting in Superman: Doomsday (I thought he was superior to George Newbern), but Tim Daly is far and away the definitive voice of Superman.
George Newbern and Tim Daly sound exactly alike in my opinion.
How is that an opinion? It's a fact that they don't sound exactly alike. It's like if I said that Tim Daly and Kevin Conroy sounded exactly alike. You'd tell me I'm wrong, but no, it's my opinion. /sarcasm.
If you can't distinguish between those two incredibly distinct voices, I have trouble giving your opinion on voice acting any true regard.
As it stands, Tim Daly is the voice that best captured Superman's wit and optimism, but he also proved himself worthy in his ability to convey many other forms of emotion.
The only reason George Newburn sounds like Tim is that he TRIED to sound like Tim to help mask the fact that Tim was no longer the voice of Superman...and he failed, miserably. While there are a few similarities between them, Tim's voice is far and away superior. Newburn was stiff and boring, while Daly had a warmth and regality, yet humor to his voice that perfectly suited this version of Superman.
You know it's funny everybody likes Tim Daly as Superman but at the end of this series he turns into a wimp. Lois Lane gave Superman a pity kiss and he smiled with glee.
They powered him down in the first season of JL so as to give the rest of the league something to do. When Timm/Dini realized they overdid the depowering they powered him back up to normal for season 2 and beyond.
As for Tim vs. George, I have to say that when I first heard Tim's voice in S:TAS I loved it, and in many ways its still my favorite, but to Newberns defense once season 2 of Justice league hit I felt he really started to bring something new (and good) to the Superman charectar instead of just trying to copy Tim Daly's (which is what I felt he did for season 1 of Justice League).
I thought Tim Daly was the best of the Superman voice actors, though George Newbern got better as he went along and Adam Baldwin & Kyle MacLachlan certainly threw out good voices for him as well (I actually consider Kyle #2 behind Tim).